NO!!! DON'T DO IT!!!
Can you talk to the previous owner/mechanic about it?
How long has it been doing this?
Did it do it when you test drove it before sale?
How long ago in miles did the trans get replaced?
Did he do the rebuild?
Definitely persue the warranty on it if it is a reman unit as it should transfer to you or at least you should be able to get the previous owner to claim it for you.
If he is a real mechanic in business, he may wind up doing the warranty work, but find out if the warranty work can be done by another mechanic as I always am suspicious when something isn't right and you have to go back to the guy who sold it. This could blow up and become a nightmare.
I'd also recommend finding a trusted, non-chain shop that handles transmissions for a second opinion. I'm just not a believer in chains.
Does anyone think a full fluid exchange and filter change will help?