You don't think that maybe your tranny oil cooler is somehow not working
properly.  This would allow the tranny oil to get hotter than it is supposed
to and then maybe expand to the point that it is pushing out around the seal.
Either that or maybe a vent in the tranny is plugged up allowing pressure to
build up inside, thus pushing past the seal.  Sounds like whatever the
problem was, that the PO had it to and that the new seal on the torque
convertor only had a temporary fix.
Hope this helps.

Ken Wilford
http://www.vanagain.com
John 3:16
Phone: (856)-765-1583
Shop:   (856)-327-0027
Fax:     (856)-327-2242