losing drive when getting hot

[doin_it_dirty]

not long had my surf. its a 3vze, v6 auto, 91, dun just over 200ks, stock standard, though i bought it cheap as from family member, and it turned out to be rather tidy, and im planning on spending some money on it. now as of late when im out in the mud spinning the wheels and getting dirty, i lose my drive, as in when im loseing traction constantly for say 1 min straight, the whole drive train stops, showing symptoms exact to that as being in neutral, (all revs, no motion)
then i leave it for say a minute or so then i have drive again. get the wheels spinning for a bit, then slowly i lose drive, then again, symptoms exact to that of being in neutral. then leave it a minute and shes sweet again. no noises, no other symptoms. If its a major a may just sell it, as otherwise it goes sweet, just plays up when you get the wheels spinning for prolonged periods, I recently had the trans pro flushed, hooked up to the machine, so gearbox oil is new and sweet.
What do yas rekon???