Ok, thanks for the new information.
Possibility 1: The pump is partially jammed and this is slowing the motor so that it cannot turn fast enough to spin clothes with a heavy load. The test would be to put it back together with the pump off, and a heavy load and see if the problem is solved. If so, replace the pump.
Possibility 2: If the pump is not hindering the motor, and the motor is turning ok and the transmission is working and the brakes are not dragging then it has to be the clutch. In this case a new clutch kit should fix you up.
One final check you might make is the drive block but it does not seem likely.
Let us know what you do and how it turns out.