I have a 54" mower that still runs great but after about 15 minutes when it gets warmed up it won't move forward or backward. Engine and Mower are still running strong. Tune up and new belt didn't help. After turning the mower off and letting it sit for 45 minutes, it runs great again until it heats up then once again stops. I'm thinking it may be the transmission which is a sealed unit. Anyone else run into this?