I would isolate the cylinder (as mentioned above) and pull the injector out and have it tested. I haven't heard of any diesel having this symptom because of a bad valve seat. That's a more common problem with gas engines.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL