This can happen for a variety of reasons including:
- pump loose on the mount... there are 3 bolts in the front and one in the back
- pump mount loose on the block
- someone overtightened the timing belt at one point and the front intermediate shaft bearing has worn (hows your oil pressure when hot?)
The first step would be to confirm the above. Some folks have been able to make small adjustments by loosening the pump and/or mount and tilting the alignment slightly, but a mis-tracking timing belt on an IDI is usually a sign of a problem somewhere that needs to be addressed.
Lots of conversations on this at
www.vwdiesel.net, where is where most of the IDI crowd seem to have gathered these days.