My 1996 Ford Ranger 4cyl. automatic is acting up. i can drive the truck with no transmission problems until it warms up then at stops it clunks into second and first gears. it only does it after warming up. A local transmission shop rebuilt the transmission and it still did not fix it. A new PCM was put in and no fix. the shop has no clue and didnt charge me anything. any clues what it could be? Thanks