Hey mate,
Firstly welcome.
Secondly At around the 35 second mark when you engaged the drive you can see the belot flapping about. Are you sure you have it on properly? It could just be the tensioner is not correct OR pulley bearings are knackered.
Eliminate these issues before you start worrying about the gear box. It's POSSIBLE the shaft is bent and therefore the belt isn't moving truly around the pulleys BUT I'd be more suspect of incorrect installation OR pulley bearings.
Keep us all in the loop - it looks in pretty good nick!
Pete
