Okay I've done prior maintenance on my truck over the last month. I've gave it a tune up, a new radiator, oil change every 3000 miles, new air filter, transmission fluid change, new shocks, drums, shoes, brake pads, and a sealing in the back axle. I will be replacing the cat converter and exhaust for a Magna flow hi flow converter and a flowmaster 50 series. The a/c isn't working and recharging it don't help. The crusis control doesnt work. After all the work I've done to my truck so far and it still acts up. When I start my truck and start driving down the road, it acts like it is loading power and begins to slow down yet the rpm doesn't rise. When I let off the gas and go to accelerate again it seems fine but occasionally back fires. Is this a transmission, exhaust, vacume or a sensor problem? I'm not sure where to look next for this problem