Rough idle. rpms drop. misfire.

the code is for random misfire, multiple cylinders, as you've replaced the plugs and leads then I would check the coils and the fuel injection

Car idle is rough Inspection Service & Cost A rough idling engine can be caused by a number of issues, some of them are serious while others tend to be minor, but the symptoms remain pretty much the same. The car will feel rough and bouncy when the engine is running. The car will also idle below its regular speed, display inconsistent RPMs and may produce a shaking, skipping or slipping sound when the vehicle is running.
While a rough idling engine may seem to be a simple inconvenience it often indicates a deeper problem within the engine. The vehicle should be inspected and repaired as soon as possible because small problems have a way of turning into expensive repairs. How this system works: The idle speed of an engine is basically the rotational speed the engine runs on when it is un-coupled from the drivetrain and the throttle pedal is not being depressed. The idle speed is measured in the revolutions per minute of the crankshaft.
When an engine is running at idle speed it generates enough power to smoothly operate equipment such as the water pump, alternator, and power steering but not enough power to move the vehicle itself. A passenger car will usually idle between 600 RPMs and 1000 RPMs. A properly functioning idle should run smoothly without skipping or slipping. Common reasons for this to happen:

• Dirty Fuel Injectors: The fuel injection system injects fuel into the cylinders, which creates a mix of air, and fuel to ignite and burn. Fuel injectors have tiny nozzles to spray the fuel into the cylinder and they can become clogged over time. clogged or failing fuel injector creates a lack of fuel in the vehicle's engine. This can cause a rough idle, it can also cause symptoms such as slow acceleration or the car not feeling as if it has enough power. If the problem is addressed early, it is possible to clean the injectors, which will restore them to full function. If this condition is not addressed in a timely manner the injectors will need to be replaced. Incorrect Idle Speed: While the average idle speed falls between 600 to 1,000 RPMs, if your vehicle is experiencing a rough idle it could be due to an incorrect idle speed setting. A trained mechanic can easily adjust the idle speed, and it should stay at the proper speed. If an adjusted idle speed becomes inconsistent or changes at random intervals there may be a bigger problem that needs to be explored. Vacuum Leak: If the vacuum system has a leak, it can seriously affect the ability of the vehicle's computer to regulate the air to fuel ratio. This can lead to a rough idle and if the problem is not addressed the car may experience slow acceleration and a lack of power. Vacuum leaks should be inspected and repaired immediately. Incorrectly Installed or Damaged Plugs: Spark plugs are responsible for creating the spark that allows the vehicle to burn fuel. If spark plugs are improperly installed or malfunctioning, the idle speed can be affected. The vehicle's engine may vibrate or there may be slipping or straining sounds coming from the engine. Defective or Clogged Fuel Pump: A rough idle can be related to fuel delivery issues. The fuel pump, which is responsible for pulling fuel from the gas tank to the fuel injectors, can become clogged or defective. If this happens the engine will not get enough fuel, which can cause a rough idle, sputtering, stalling and even slow acceleration. Clogged Fuel Filter: A clogged fuel filter can cause similar problems. The job of the fuel filter is to screen out contaminants in the fuel, over time it will become clogged and need to be replaced. A rough idle is one symptom of a clogged fuel filter. Failing Electrical Components: A problem or failure in the ignition system or various electronic components can cause a rough idle. If this is the case, the problem will usually get worse as RPMs increase. Common culprits include the ignition control module, plug wires, coils, and spark plugs. Defective Airflow Sensor: A defective airflow sensor can be responsible for a rough idle. A mass airflow sensor detects the amount of air coming into the fuel injection system and sends that information to the vehicle's computer. The computer uses that data to deliver the proper amount of fuel to the air in the vehicle. Over time these sensors can malfunction or become dirty. One of the first symptoms of a malfunctioning airflow sensor is a rough idle. The car may also accelerate slowly and even stutter or stall as the problem progresses. Dirty Oxygen Sensor: Oxygen sensors measure how rich or lean the gases are as they exit the combustion chamber. Depending on the results, the amount of fuel entering the engine is adjusted by the vehicle computer. The ultimate goal is to maintain an ideal mixture that produces the lowest emissions. A dirty or failing oxygen sensor will usually trigger the check engine light and can lead to a rough idle, lower fuel efficiency and failed emission test.

Have you checked for a broken valve spring. Repaired a dodge engine with the same running issue as you have described and it was a broken intake valve spring.

You have to correct the cylinder misfires first

You may have vacuum leaks somewhere in the
intake or exhaust

You really need to look at data with a scan tool

You start with Mode $06 & cylinder misfire count ,if that
even works well on your motor

Then you look at fuel trim & add short & long together,
to get an idea if it is running rich,which would be a lean
condition from a vacuum leak or rich from leaking injectors

All the data is only good if you haven't cleared it or disconnected
the battery. If you did, then you have to drive until all I/M Monitors
run & then look at data

You look at O2 sensor data & even scope them

You made no mention of checking for codes
No mention of a cylinder leak down test for valve leaks
Exhaust Converter age or condition !
Fuel Pressure !

You have an air leak around the throttle body

I have a malibu 3.1 and it idles very good at about 650 rpms, when warm. it sounds like your problem may be a bad coil. a multible misfire can be-- wires, coils, vacuum leak in a hose. but change fuel filter and check the coils one may be bad. i think at auto zone they are a reasonable cost. GOOD-LUCK!

You could have a clogged fuel filter, if you haven't changed it in awhile. You could have a leak in the intake system somwhere also. When you slow down, you get less power to the engine. It may need spark plugs and wires as well, if in need for a tune up.

You may need to check the engine timing, that would be the first thing,, 4.3 is also bad about leakes in the fuel lines that are under the intake manifold. this is part of the fuel injection system, it will cause it to rough idle, but will accelerate fine, but when come to stop, will have a rough idle.

try fuel injection first and drive for awhile to see if it is a dirty injector,,,

The air fuel mixture sensor is probably starting to fail. You need to have a simple diagnosis done at the shop and then have it replaced - they are usually not much a reputable mechanical shops. When the motor warms up your air/fuel mixture changes hence the rough idle etc. Hope this helps.