Hi, Steve - - -
While not familiar with your car, I've experienced an interesting diesel problem in the past that might be related . . .
Background: I was living in the San Francisco Bay Area and experiencing a rather lengthy commute each day. I switched from a Mazda RX-2 (Wankel rotary) to a '78 Rabbit (Golf) 1.6L diesel banger - going from 14 mpg to
42 mpg in the process. This was right at the lead edge of a major oil hassle that didn't much affect fuel costs, but raised hell with availability. Thoroughly pleasant little car that I condemned to a slow death with the installation of aircon. At that time, I was offered the job that brought me to move here to Reno.
"The problem:" In the course of a weekly commute from a job and apartment in Reno to the bosom of my family in California, I was hitting the freeway hot and heavy. On two occasions, during hard runs on hot summer days, the car just took off on its own. Cut the ignition and accomplished nothing. Had to pull over and stop without declutching to kill the engine. On one occasion, while at idle, the engine began a very ragged acceleration on its own.
The fix: Not all that comfy with a diesel, so took the car to the VW Dealer in Reno, who had that very week received a recall notice from the factory. It seems that there was a problem, only apparent under high temp operation, where engine oil on its way to recirculation into the combustion process would condense and "pool" in the plenum chamber. There was a drain, but too small. Under continued high temp operation, this pooled oil would vaporize again and flow into the engine, which treated it as crappy, but burnable, fuel. A simple plumbing change solved the problem.
Suggestion: You have this possible source. You also have the possibility of too much oil leaking in the turbo, with similar results. So, you get to check the plenum for signs of oil collecting (hoping that this is as far as you need go) and/or looking at the turbo as a source in need of a rebuild.
Good luck.
bob noble Reno, NV, USA