Repairs 72" cheetah deck belt jumps off

first post here being that my mechanic cant seem to resolve my problem. my 72" cheetah's big deck belt keeps jumping off my mower ever so often when you engage the pto. I have checked the idler arm bearings and all the pulleys seem to spin fine with no slack but it makes a loud screetch when engaged and the belt looks real wobbly. my throttle position sensor has been replaced but my mechanic has not adjusted the rpm cause he doesn't have any diagnostic equipment. I was wondering if that could be the problem beings that the rpms may be lower than its supposed to be. thanks in advance for any advise

Belts jumping off with a screech is normally the tension is too low. The other issue is the tensioner pulley will not be level with the rest of the pulleys. I alway check with a straight edge.