bucking

I have a 00 Accent and my problem is as followed, while driving it feels as though the motor is going in and out of limp mode. ill be cruising at about 2800 rpms and it will like loose power and as soon as it does it comes right back causeing a jerking or bucking feeling and every now and then a back fire pop in the exhaust just depending if i let off the gas when it happens or not, i do have a check engine light but never fails it goes off before i can make it to the parts store to get it read. it also does it sometimes when accelerating but hardly ever after you get the rpms up past 3500. now the things ive dont to try and fix this are, replaced the spark plugs, plug wires, and coil packs, i dont think it matters but u have also changed the water pump and thermostate, reason i dont think it matters is that it was doing it before i changed it. can someone please help the only other thing i can think it might be is the fuel pump.

Thanks in advance to anyone that can help!

 

in the past year ive put a new crank sensor , iac , tps , maf , both O2's , plugs , wires , coil pack , thermostat , and water pump

problem started when i changed back to factory thermostat from a 170 degree one so the check engine light would stay off since the thermostat ive changed the water pump, plug, wires, and coils i even went back and put the 170 Degree thermostat back in just to check bad thing is i have poly urethane front and rear mounts and i went and filled the voids of a transmission mount with poly urethane so every little hiccup i feel and this bucking is driving me crazy. the other thing i thought about was maybe my trans i have the 5 speed manual and the diffarental is going bad again from where i slammed it (ive raised it back up some)

 

it was the maf