How do I stop my Flymo 350 Glider from continually shredding its belt?

The belt on my Flymo 350 Glider keeps shredding itself after about 10 minutes use of the mower. I've checked the pulley wheel from the motor and that seems free moving and all in order. The other larger black plastic wheel which drives the blade is not as free in its movement and does not spin freely. Should it be like this or is this the problem causing my belts to break???

Jeff
June 2012