that is a very common issue with those. my 91 does the same thing. you might try changing the transmission fliud. it seems to help mine for a while.
i have heard that some people use a high performance oil, but i cant think of the name of it right now. i use BG multi-gear concentrate in mine. if oil change doesnt fix it (you probably have a leak too) you may need to take it in to the shop. people that really know how to rebuild those tranny's are getting scarce.