1990 3.0 LS V6 Auto 200k

My mechanic can't tell me why it won't shift into overdrive unless I've heated it up quite a bit. He suggested taking it to a transmission place, which I did. They said that they'd have to open it up to see if it was the trouble, which costs about the same as rebuilding it.

Once it does shift into overdrive, it drives fine, shifts fine, until I stop long enough for it to cool off.

So it doesn't make sense to spend $3000 to rebuild a transmission that may or may not be the problem.

Which means I might have to sell it - as much as I hate to. I wonder if I'll get more than $1000 for it?

If anyone has any suggestions, please let me know.

Thanks,

Ken