Try to find a parts manual or serv manual on that machine - a lot of the hydro trannies built 15 yrs ago were of the "serviceable" type, meaning you might be able to drain/replace fluids/filter - might really help that old tranny.

M/W, don't forget to closely chk your ground drive belt and driven tranny pulley for splines not damaged/etc.

but, the fact that it will do OK until tranny warms up speaks to tired old oil, low oil and mebbe internal pump probs (that's when you hit the $800 barrier...)