You might also want to check the deck brake is working right. It takes a surprisingly small amount of contact for the belt to still spin the deck even with a very loose belt. On my model the brake is a bar running across the chassis at the "firewall" with a brake pad mounted in the middle. When the deck comes up it presses on the top of the drive pulley. It is adjustable by loosening the mounts on either side. The pad may be worn out in which case you can purchase a replacement pad from outdoorking very cheaply.
