Hyundai Repair: speed sensor?, presures, raditor

Question
i bought a 2001 accent in december of 05. now its may 06. in april my engine light came on so i took it in for a tune-up. after the oil, transmission, raditor and everything was replaced the car was running fine. about a week later that light came on agian. i took it to auto zone and they hooked up this thingy that said why that light was on. the results said about not enough input voltage or output voltage to the speed sensor, and their was a bad combination in the 4th gear. i dont know what next to do cause everyone ive asked has given me no help. when i drive the car it will be fine but after 15 min or so of the engine running the car seems to loose some accleration. one mechanic said it sounds electrical and another said try some fuel injector cleaner. i dont wanna do anythin untill i know exactly wats wrong. and how much its gonna cost me. thanks

Answer
I cant say how much but it sounds like your having some trouble with the trans its self from what your saying,like the engine is reving up but your loseing power sounds like the trans is slipping but with out knowing the presures of the trans i can say so sorry