Bought a 98 forester just a week ago with a small miss in engine today started mis -firing badly, differcult to accelarate but then providing not heavy footed would cruise along nicely at 65-70 mph, took it to garage who changed plugs then said they were not that old, but could not be any more help.what else should i try, sprayed the leads with wd 40 today also.

usually a secondary ignition problem(coil pack, spark plug cables).

Sounds like a basic cylinder miss-fire. Common issues are spark plugs, wires, coil, and fuel injectors,
is the check engine light on when the engine is running ?

remove the plug from the #1 cylinder, and install it into the #2 cylinder , and put plug from # 2 clyinder in to #1 plug hole, connect plug wires, and start up vehichle, and if computer shows misfire on # 2 cylinder , then you have a bad plug, possibly porcelin insulator could have a hair line crack in it, hope that this information has helped you.

Sometimes you can use digital multimeter to check for ignition mis-fire.
With engine idling, touch spark plug wires, where applicable, one at a time and note the reading. You don't take the spark plug wire loose and you don't penetrate the insulation, just touch it. Some of the readings I've seen around .6 or .7 volts, not even a full volt. If you find one or two that are bouncing off zero, then you backtrack those. It's an inductive reading or whatever you want to call it.
Hopefully the misfire code will tell you which cylinder, then you check fuel--spark--compression on that cylinder. If p0300 mis-fire code, multiple cylinder misfires and the system can't determine which cylinder.
You might have a vacuum leak?
The basic for a rough idle, you check for a leak of some kind, fuel--egr--vacuum.

On my 2002 safari I had a inter miss fire. After double checking all electrical. I had a bad injector of all things that was bad. My not be ur case but I thought I would mention it

P1505 and P1507 refer to the Idle Speed Actuator

Hello,the problem there is not a good spark in the compression chamber,so that it will develop a bad miss firing and you will not have a good acceleration.I advice you to change change all the spark plugs wire ,the wire lose the spark.Hope that help you

trottle postion sensor

there may be a software update for the powertrain control module for mis-fire monitor. check with your local dealer. but if thats not the case,these engines are known at least in my experience of having the heads crack between the valves and cause mild mis-fires. try compression testing all the cylinders and performing a leakdown test on them as well.

A BLINKING ENGINE LIGHT INDICATES AN ENGINE CYLINDER IS MIS-FIRING. HAVE THE TROUBLE CODES TESTED TO DETERMINE WHICH ONE. DO NOT DRIVE THIS WAY AT HIGH SPEEDS AS IT MAY OVERHEAT THE EXHAUST SYSTEM AND CREATE A FIRE HAZARD.