Try going up the hill first thing when the mower & tranny is fairly cold then try again when it is nice & hot.
Pulls up fine cold but slips when hot indicates the tranny is on it's way out.
Some times draining and refilling with a heavier oil will buy you a few more seasons , sometimes not.
IF it slips both hot & cold double check all the pulleys and in particular the drive pulley on top of the tranny which can flog out the splines so slip under load.
