if you can rule out the belt and linkages to the trans then it is probably
internal. before you go and rip the thing apart try changing the fluid, if
it is low or contaminated it can effect the performance. If you change the
oil and know for sure the belt and linkages are not the problem and the unit
still has low or no power (or loses power as it gets warm) then the pump may
be damaged, at that point I would take it to a pro and have it check.