Hi, the belt is erratic this indicates problem with DC board or the motor. Manually push the belt in the direction in which it normally moves. If you have difficulty turning the belt, there may be an issue with the belt/deck friction. If the belt turns freely, there may be an issue with the DC board output.
Use a multimeter to test the black and red lead wires that connect the motor to the power supply. An external power cord connects the motor the power supply.
Most probably its a motor issue.Its got weak,because in most of the cases its seen.But in some cases the board is checked faulty.-------- Get the motor as well as board checked to confirm the fault. This will help