Like Jack says, check adjustment first. Especially pressure screw adjustment.
If the pressure screw behind the derby cover has the correct 1/2 turn of lash when cold, you may try changing the diaphragm spring. It is item #3 in the attached pdf. That is the spring that does all the work. If it gets weak from heat fatigue, the pack will slip. Easy and cheap to replace.
You did not mention the type of fluid your using. I personally would use formular + in the clutch pack area.