loss of drive?

have jcb 3cx all elements work fine but have a problem with drive works ok with good power for about 10/15 min then complete loss of drive leave to cool and will work again for 10 min or so .
not sure what oil to put in have drained and cleaned bottom screen and changed filter nothing untoward found screen clear.
also do i check oil with engine running as this is what we have been doing allthough I now think it may have been overfilled would this cause loss of drive ?