TH 400 Transmission Leak When Parked

[72step]

Last year I bought a seal and gasket kit for my tranny because it was leaking on my garage floor. I replaced every seal I could without taking the tranny off and rebuilding. The tail housing seal, dipstick seal, tranny pan, speedo seal etc... It remedied the leak for about a year until it suddenly leaked fluid onto my floor again. Apparently it's not leaking too much because I bet I could go out there and put a quart or 2 and it would run fine.
The thing is when I put fluid and go for a drive is the tranny works like the day I got it rebuilt. It has a TPI racing kit in it so it runs great.
Now if I take it out and in to a shop, would I be best just doing a complete rebuild? Or could it be a certain seal? I guess my best bet is to buy a complete Performance rebuild kit and take the tranny in to a shop. This is getting old, because this is will be the second time I have to rebuild it, as it started life out as a TCI racing tranny I bought from Jegs some 11 years ago, then around 1999 I had to have it rebuilt because it was shuddering.
Are TH400's known to do this?