The Mellotron Group

Hi all, 

recently my studio was given a soundsales mellotron. Unit worked great, new motor card installed, clean headblock, no pinch roller or pads missing. Ran well. However, as of today for some odd reason either the motor is stalling under the load or the drive belt is slipping. 

I take the motor out and use the pitch control and it responds, when i have the motor in the unit and use the pitch control the flywheel goes through this "driving up a hill motion", slow going up then fast coming down. Ive tried adjusting the tension on the motor and itll latch on but then the pitch control seems to not do anything? I believe it to be the belt since it feels really hard and smooth. Any ideas? How can i test the motor?

Ive tried contacting Markus but have yet to hear back, anyone know where else i would be able to find a suitable belt replacement? thanks!