96 cav no power

96 cav 2.2 the motor act's like exhaust is cloged up,run's at half speed then stumble's. the only code's I get is a po300, that's a miss fire. let me tell you what I have replaced on this car from hell.
took off exhaust and ran it made no differance,replaced wires,plugs'..both coil packs,..I had even taken the box that the coil packs are bolted to over to advance auto had them check it out with their new machine that heat's it up....they said it was fine,went back and put it all back together ,and it still ran like crap,so I checked fuel pump pressure,60 psi at pump went down to 35 and was stedy,changed fuel filter,replaced the pcm,tps,crank shaft sensor,I even drained kuel from tank and replaced with fresh fuel,,,what else can I do,before I roll my wife's sister's car in a ditch some where......

it does it when the motor is hot or cold

Check the valve timing, may have skipped a tooth or two. Retarded timing feels just like a plugged exhaust.

yes that was my next move the motor has over 100.000 and my wife's sister drive's the car like a bat out of hell I'll have to pull the valve cover off and check the timing,maybe monday I'll post with what
i find

check the conector for the tank that goes up through the floor, mine was intermittent and I cleaned that plug hasn't come back.

I found the trouble,turned out to be the timing chain sliped about 5 teeth, it did not back fire at all .I do not have it started yet .will repost when I do.....

I did start start the motor ,and it run's like a new one,it has 133,000 miles .so if any of you have a cavalier with lack of power,with that many miles on it,act's like exhaust cloged.check timming.......