Sounds like a plugged cat to me (if it still has cats on it). When you drive around with a plug wire that isn't firing, it is sending alot of your fuel and crud into the exhaust stream and it tends to melt the platinum in the cat over time. Take your exhaust bolts loose where it bolts to the catalytic convertor and just leave the exhaust hang down and go drive it. If your power comes back, then your cat was plugged and you need a new one. If your power still isn't better, then the most likely cause would be a tranny issue (probably torque convertor problem).

 

pull the computer out and make sure the harness, i believe its the one that says "grey" is completely connected. mine was doing similar things, and it turns out the shop broke it and tried to glue it back in.