TDI Starting & Running Problems: GIII 90 PS

Hello. My TDI has been having starting problems for a few days. It's been cranking for minutes with white smoke. Since yesterday, I've also been experiencing problems after coasting; the engine doesn't rev up anymore, and blue smoke is the result. After releasing the clutch and the engine speed drops to idle, it revs up freely again, without any loss of power. I had the error codes read, and it showed error code 550: fuel injection timing control - control difference. The car is from 1997 and has almost 267,000 km (approximately 166,000 miles) on it, with no previous problems. The mechanic at the workshop said it's likely the fuel injection pump and that it needs to be removed and sent to Bosch for inspection. Do you agree with this assessment, or does anyone have another idea?
Okay, fuel filter drained, new glow plugs, air filter is okay.
The injectors are still the original ones.
Thanks in advance!

TeeDeeIii wrote:

or does anyone have another idea?

Yes:
1. I'm turning this thread into...
2. Do you remember the search function (target area: articles) that was presented to you during your registration, but which you apparently skillfully ignored?