99 Pathfinder Mysery problem!!! Need HELP!

[budigroove]

I need a diagnosis, but don't bother unless you have a code reading device, we had to take apart the dash to get the codes manually. We got a code for the Knock sensor so we replaced that, the plastic on the sensor was melted! There was another code for a multiple cylinder misfire but that went away. Then no code but still broken. We ran a test on the distributor and it failed so we replaced it. It ran very well for about 4 miles then started heating up again. It runs like a kitten when NOT hot. This 99 Nissan pathfinder doesnt idle right. I changed the timing belt, it originally skipped a tooth so we straightened it out. We put a new water pump, new intake gasket and plenum gasket I believe. We took the muffler off before we changed the distributor in case it was the catalytic converter, (there might be two converters we never checked the one next to the engine not sure)...but it still ran bad so we had it ghetto rig the muffler back on. I still need to weld it...this car is a big mystery and a big headache. I still think its a converter cuz when it runs it feels like there is something holding back and it doesnt idle well after driving it for awhile. One time the check engine light came on and i stopped driving it, but for the most part it drives very well at first, then craps out later. It idles at about 1500 RPM when hot, then it slowly goes down to 1000. And it cycles like that from 1000 to 1500 RPM. the idle helps propel the car when not pressing on the brake at about 15 mph. please help! I need to know whats wrong i bought the car off a friend hoping to fix it but its only at 75%

Thanks,

Rudi

[fueler]

have you changed the spark plugs? tried cleaning the MAF sensor with electrical contact cleaner?

[JonathanPrem]

have you changed the spark plugs? tried cleaning the MAF sensor with electrical contact cleaner?

Cleaning MAF doesnt work- i learned it class. You basically adding chemical that stick to the thin wire and once it burns its give bad reading and also builds up

[budigroove]

the spark plugs are fine. i checked them and theyre cherry