Power loss up hill

Hi guys, I have an elderly 22hp 50" cut Yardman that runs well. I got her out to do the lawn this week and with the pto engaged it nearly stalled going up a small hill I have in the back yard. I disengaged the pto and she flew up the hill no problem. This also happens when I hit the longer grass. I removed the main drive belt, and all the blades rotate easily. Any suggestions, as I'm at a loss.
Thanks, Rusty.:banghead3