i have an 87 gmc sierra 4x4 half ton. i put a new tv cable and its properally adjusted and i by passed the tps so the tc lockup gets 12v unless the brakes are apllied. the truck runs and shifts fine, overdrive and everything until it warms up and you allow it to shift outta overdrive. then it wont go back in. downshifting to 3 does nothing hence the no overdrive. what could it be? can i fix it myself? what can i try and do? money is tight due to awesome tuition fees (sarcasm) so a tranny shops outta the question.