major problem with accelerating :(

[Stang39r]

i hope someone has at least had this problem before. here goes:

- dramatic decrease in power and difficulty accelerating at all when using more than ¼ to ½ throttle.
- The symptoms are at their worst when the car is still warming up and not at operating temperature (1st 5-10 minutes).
-A handful of times I have experienced what seemed to be a small backfire or pop from the exhaust.

i have replaced everything i can think of (O2s, all the filters). the car seems to be running really really lean because of the sputtering that happens. when i floor it in nuetral the car doesnt respond right away and when it does it jumps in rpms and not constant. one of the only things i can think of is my computer not working right. i disconnected for awhile to see if i could reset it and it be normal again but no.

i recently installed a new o/r h-pipe and that has been the start of all my problems. i dont see why. i can understand a little less low end by the loss of back pressure but thats it. the car shouldnt act like this. i hope someone can help me cause i cant even take the car out cause i fear for the engine and the fact that it hardly moves thanx