Sounds similar to my only 'problems' on my 06' 42 incher. At about the 20 Hr mark it jumped off. Over the next 5 hr or so it came off couple more times again...ate belt/streched belt by that point..bought new one from dealer, replced..mowed a few hours jumped a few more times over a 10 hour period...destroyed belt again. Was at point of taking entire machine 3hrs to Dealer
but I had bought 2 belts last time at Dealer. Put I put the 3rd one on and its been running 'Jump Free' for last 50 hr's or so....knock on wood. So maybe the pulleys, spring, Idler has to 'break-in'..who knows...
But I'm Happy now...
By the way..pay-attention to your 'Tension' setting on your chain/Idler when you replace Belt, It plays big part in Blade RPM, overall cutting quality.