I had the exact same problem with my TD, it was an increadibly frusterating month of diagnoisys b4 i finally figured it out. I tried everything i could think of and then things that others thought of, what it ended up being is that some where along the lines the plate that holds the injection pump, the one that is on the timing belt side of the motor, not the support bracket under the pump had some how become tweeked and caused it to run off. The only solution was to replace that bracket. I tried straightning it, but it was out so little that it was hard to tell if it was straight or not.

I ended up just using another one off of a car from a junkyard and presto no more problem's this was after 3 timing belts and like a million tries. To this day I still have no idea how it got bent, or what caused it, but that is the solution.

I think the most common cause for this is the injection pump shaft bushing getting worn. Just because it's old or because the belt has been tightened too tight sometime in it's life.
 This causes the shaft to ride at a slight angle and the belt runs off.

I had the exact same problem with my TD, it was an increadibly frusterating month of diagnoisys b4 i finally figured it out. I tried everything i could think of and then things that others thought of, what it ended up being is that some where along the lines the plate that holds the injection pump, the one that is on the timing belt side of the motor, not the support bracket under the pump had some how become tweeked and caused it to run off. The only solution was to replace that bracket. I tried straightning it, but it was out so little that it was hard to tell if it was straight or not.

I ended up just using another one off of a car from a junkyard and presto no more problem's this was after 3 timing belts and like a million tries. To this day I still have no idea how it got bent, or what caused it, but that is the solution.

Are you talking about item #4 in this ETKA excerpt?

HermTDI your pic isnt working, so I can't answer that.

As for the rest of the answers or questions, My pump was fine, worked great untill that, worked great after the bracket swap, I triple checked the alignment of the botls, there tightness the tightness of the timing belt ect, I literally worked on this for a month... all to no avail. It was only once the bracket was swaped did the belt stop walking off the pulleys.

I recently pulled my in injection pump to convert to non-AC and found this:

 Good way to find this out and the new bolts got some blue locktite on them.
 The belt still wasn't riding off the pully.
 I have had them ride off the pully before and not leak or suck air.

 I went to look at an '85 Golf diesel. When I went to start it there was a bad belt rubbing on the timing belt cover kind of noise. Took a look and sure enough ... I quick shut it off and told him about it.
 I told my friend about it and when he went to check on it, the price had gone up conciderably.
 I learned the main cause for this problem and went back to let him know why it was running off. He had taken it to the dealership to get the belt changed.
 I told him they didn't fix the problem and if he checked, it would probably be running off the gear again. Next time someone checked on it, the price had gone back down.
 It's on my short list of cars to buy, fix and sell ... or have one of my friends buy it and pay me to fix it.
 So many VW diesels so little time.