When I go 50 to 60 mph the engine stutters what could i Could it be the spark plugs? Or Could it be the catalytic converter?

Improper Ignition Timing

Defective Ignition Components
(ignition coil/s, distributor*, distributor cap*, distributor rotor*, ignition wires, and spark plugs - NOTE:*Distributor-less engines do not have these components)

Lean Fuel Mixture

Defective Catalytic Converter
(Some symptoms of a bad catalytic converter:
Major loss of power over 15-25 mph.(does this speed sound familiar to you?)
Strong sulfer or rotten egg smell emitting from the exhaust.
Loud rattle being heard from inside the catalytic converter.

Defective Air Injection Components

Faulty smog pump if fitted.

Low Cylinder Compression.

black smoke equals too rich a fuel mixture. check the engine vacuum at idle should be 20 inches of mercury if not check for stopped up catalytic converter. are there any check engine lights

the main symtom of a bad converter is. you can push pedal to floor and only go about 15 mph, if it is clogged up. the car will idle well but no power at all. the fist thing is overheating usually from misfiring spark plugs, and burns out the mechanisms inside. Catalytic converters are not designed to handle the heat of burning fuel; when fuel starts to burn in the converter, it can cause a great deal of damage to the converter and the rest of the exhaust system. One of the noticeable symptoms of overheating in the converter is a significant loss of power from the engine.

CODE PO420 CATALYST BELOW NORMAL BANK 1 .POSSIBLE CAUSE CATALTIC CONVERTER , WIRING , HO2S 2 HAVE VECHICLE PUT ON A SCANNER TO CHECK FUEL TRIM.A RICH FUEL MIXTURE WILL CAUSE THIS PROBLEM.MAKE SURE SPARK PLUGS AND WIRES GOOD AND FUEL INJCTORS ARE CLEANED NOT CAUSING ENGINE FLOODING MAKE SURE O2 SENSORS OKAY.IF YOU HAVE TO REPLACE THE CATALYTIC CONVERTER. AND FUEL SYSTEM STILL RICH IT WILL BURN OUT NEW CATALYTIC CONVERTER.YOU WILL BE REPLACING IT AGAIN.

It may mean that one or more of the following has happened:

•Faulty spark plugs or wires
•Faulty coil (pack)
•Faulty oxygen sensor(s)
•Faulty fuel injector(s)
•Burned exhaust valve
•Faulty catalytic converter(s)
•Stuck/blocked EGR valve / passages
•Faulty camshaft position sensor
•Defective computer

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

The underlying cause of random misfires that jump around from one cylinder to another is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Your cat should last quite a while now it is replaced. I recommend testing coil packs and module. Change plugs and wires and double check your vacuum hoses.

change the pcv valve its under the intake manifold

it sounds like the cat is blocked,

Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s). in this list u will find a answer to ur problem Shaun 2 Inspect Spark Plug Fouled, damaged or broken spark plug(s). 3 Inspect Fuel Filter Clogged or dirty fuel filter 4 Inspect Distributor Worn, loose or incorrectly adjusted distributor. 5 Inspect Distributor Cap Loose or cracked distributor cap 6 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor. 7 Inspect PCV Valve Plugged or damaged pcv valve. 8 Inspect Air Filter Restricted or clogged air filter. 9 Inspect Catalytic Converter Clogged, damaged, defective or faulty catalytic converter. 10 Inspect Automatic Transmission Improperly adjusted kickdown cable adjustment or internal transmission problems. 11 Inspect Fuel Pump Improperly functioning fuel pump or circuits. 12 Inspect MAP Sensor Improperly connected or faulty m.a.p. sensor. 13 Inspect Mass Air Flow Sensor Improperly connected or faulty mass air flow sensor. 14 Inspect Throttle Position Sensor Faulty throttle position sensor or throttle position sensor circuit. 15 Inspect Timing Specification Incorrectly adjusted engine timing. 16 Inspect Fuel System Pressure Incorrect fuel pressure being delivered to carburetor or fuel injection system.

have you the intake off? if so check for trash in the fuel ejectors. I had the same problem, I just replace the fuel meter body with one from the junk yard