i have been restoring a 1971 Nova ss for past three years. during the tear down stage i sent th350 out for rebuild and beefing up. as part of the rebuild a shift kit was added.  the 350 block was bored 30 over and trw pistons installed. a cam at 480/480 237 duration was installed with stock heads and holley 600 carb. once everything was installed it seem to work fine.  th350 was hitting real good. biting from low to second.  however it has now developed problem in that it doesn't change auto but has to be done manually. feels sort of like power glide i had in 1968 chevy II years ago.  also nolonger has bite in the shift. i've not dogged it.  what seems to be the problem? please help. i don't want to put a lot of money in another rebuild that may not last.  

Sounds like a problem in the valvebody. First check the vacuum solenoid and make sure it's not toast.

Then I'd say you need to drop the pan and check the valvebody out. Could be a check ball or spring is fubar.