engine stumbles and looses power

I have a 97 Passat with 103K mi. It ran fantastically for the first thousand mi. then quickly deteriorated. There are no engine lights on and it idles as well as ever. The problem is when a load is put on the engine. When the engine is cold it stumbles, recovers then stumbles, until it warms up. At this point it is not so much an actual stumble but like turning on and off the power. It will go along for several seconds on more power then the power is noticeably less. When the power loss comes there is a distinct buzzing sound that can be heard. When the power returns the buzzing stops. I dont believe the car has nearly the same power as before the problem occured. I am so new to TDI's that I really don't know where to start. Any help would be greatly appreciated.

When was the last fuel filter change?

The buzzing might be a problem with a vacuum leak (not related to the filter)

I changed it today. It didn't make any difference.

First, welcome to the club.

Second - look at all of the small "vacuum" hoses around the engine. I suspect you may have one with a hole in it - especially if the "buzzing" could be air loss. I'd check the one going to the ecu very carefully.

Thank you for the welcome! As I searched further this morning I came across the post concerning testing the maf sensor by unpluging the sensor. I did this and the engine quit stumbling and with the exception of some loss of power ran ok. I just installed a K&N filter last week. I read several opinions that oil based filters might increase the failure rate of maf sensors. Coincidence? I'm not sure.

The K&N doesn't do any good, and some believe it does harm. You might consider putting the OEM back on.

Your MAF may be able to be cleaned - there is such a thing as MAF cleaner. The newer ones generally don't do well with cleaning, but the older (A3) models seem to be more robust.