how is the fluid? clean, smell burnt?- most auto transmissions that show these signs are due for rebuild, problem is caused by the forward clutch pack friction plates being burned and clutch steels are warped causing forward clutches to apply when cold. as the tranny warms the clearance grows and the clutches stop applying. sorry for the bad news but time to rebuild it. this is my opinion, but after years of doing my own transmissions this probably the cause. hope this helps.
joe