Audi A6 Avant V6 TDI: Running on 4 Cylinders?

v6 TDI, manufactured in 6/98, experiencing issues when starting in the morning in temperatures around -3 to -4 degrees Celsius.
The preheating light comes on, and the engine starts right away (always on the first try), but then it only runs on 4 or 5 cylinders for about a minute. If you give it a little gas, it seems to recover a bit faster.

Who knows if it's the spark plugs or the fuel injectors that are the problem?

info: has 229,000 km, timing belt replaced 10,000 km ago!

How can I possibly measure glow plugs? Or what can be measured at all?
Measure everything that would be useful (including with a tester)?

Thank you, Harald.

Hello,

Please use the search function. A lot has already been written about glow plug tests.