Yep second what JETahh said as a starting point as it could always be more than 1 thing causing it but knowing the current IQ is important. Having access to vagcom is going to be crucial here, you can't do the hammer mod without vagcom and your going to want it to be able to check other parameters relating to your injectors and timing.
Pretty common. IQ is often cited. Factory injectors might benefit from a diesel purge.
My '99.5 had it pretty bad, but only when coasting, never stalled, I just ignored it. I ran a log of it once, fuel delivery went to 0.00.