I know this isnot a old truck but still a chevy.I have a 1990 truck 454/th400. the trans was rebuilt about 5 mo ago and has worked fine since. Then about 2 weeks ago it stuck in 1 st. drove about a 1/4 mi and it shifted and worked ok rest of trip. few days later same thing. It seems to happen after driving for a hour and then sitting awhile. Talked to the trans guy and he gave me a todo list. found nothing unusual .It is intermittent and I hate to leave it with him for to long. any ideas? Fluid is up , no leakes vac line to shift mod is good and the plug for passing gear is clean and tight.