For what it's worth, I had a very similar issue with slippage on a 700r4transmission I had rebuilt on my 91 Trans Am. It would actually kill the tranny after a while. The shop rebuilt it, using my original factory case at least 5 times. I tried to insist that they use a different case as there was obviously something terribly wrong with mine. They wouldn't budge, and I wasn't in any mood to let those idiots continue wasting my time throwing new parts in a bad case. I even sat there one time and watched them completely rebuild it to make sure they weren't pulling a 3 card monte on me, and they definately were rebuilding it. Finally I took it to a different shop, got a completely different case, rebuilt and all, and no more problem slippage, blown tranny after 1000 miles, etc. It ran like a brand new car again. Tranny is still going strong with zero issues.
I won't go so far as to say that is definately your problem, but it certainly is a viable solution to a problem you're having that isn't coming up with many alternative options. Hope you get it worked out.