White Smoke & Startup Problems

Hello,

My Golf 4 1.9 TDI AHF with 110 hp (******** chip tuned to 140 hp), with 110,000 km, has been experiencing the following (sporadic) problem for some time:

When starting the engine from cold, the car starts immediately as usual, but a LARGE white cloud comes out of the exhaust.

When I turn the car off, put it in park, and then try to start it again, I have to crank the engine 10-15 times before it starts.
Here's the translation:

'The same issue occurs when the engine has warmed up and then needs to be restarted (e.g., while stuck in traffic on the highway).'

Interestingly, none of these errors occur consistently; they only happen sporadically.
A friend of mine mentioned that he once had a similar issue with his Audi, and the Audi dealer replaced 'something' in the front of the car, possibly a valve or something similar. Unfortunately, he couldn't provide any specific details, as he doesn't know much about engines.

What could that be?

'Try searching for Hähnlein's cold start thread. It sounds similar, except that for him, it happened constantly, not just sporadically.'