1994 Colt intresting engine problem

I recently bought a 1994 Colt for a good price it has no rust on the body and it's in good shape, the engine is kind of tricky though, When it first starts up the engine runs really ruff you have to push in the clutch and give it gas to keep it going, but after it warms up the idle is way to high, the engine has 277,000kms, but all of those kilometers were on gravel roads outside of the city, the intresting part is after warming the engine up i took off the first spark plug and it was an ACdelco plug and it ran a 160psi, then second cycender ran 130, and after warming the car again I tested the 3rd and it was 120, but the fourth was only 80, also in the 4th cycender it had a diffrent plug then the other 3? Only last thing, were the headers meet with the flex pipe theres a small opening before the oxgen censor, it's a EFI, so I don't know if that has something to do with it. i've changed the oil added some helper stuff for the engine and also have added EFI fuel cleaner plus a new distribured cap. Does anyone know what I can do to fix this problem? Without rebuilding the motor?

Thanks

sounds like bad compression problem...

did you put oil into the hole and see if compresion changes compred to doing it dry ?

also you maybe want to check the MAP sensor (expensive piece).

take another good look at your vacuum hoses too