i had the same problem with mine, banged my head for awhile until i decided to rebuild it. These have a common problem with the tcc solenoid valve releasing when it gets hot or not engaging at all. apparently it has something to do with the valve wearing out and not sealing properly. The guys who rebuilt my tranny got a kit from sonnax that made to fix this problem with an oversize valve
here'* the homework the owner gave me at the time to read up on.
basically they bore it out and install an over-sized valve and all is good again.
also, when they had it all apart i got them to replace the differential with one from a ssei(apparently the se and ssei tranny'* carry similar parts), which is beefier and changed the final drive, so now i do just under 2,500 rpm at 120 km/h.