It sounds like more of a pump or pressure problem rather than the clutch plates because it is slipping in forward and reverse. Could be once the oil gets hot, and thins out, the pressure drops due to the pump. The screen that filters the oil to the pump may be just about plugged and after running a while does get plugged. Then after shutting off it clears itself a bit. Also, how about the inching valve? If it is the one mounted on the top of the transmission, they have a problem with the springs breaking. Check to see if the lever going into the trans is being pushed all the way back by the springs to engage the trans.