OK Guys, here are the updates:
This morning I went to my mechanic and we tried to figure out what's the problem. We used the information provided in this thread and this what we have done:
1. Disconnect the VBoost round connector - problem remains
2. Disconnect VBoost square connector - problem remains
3. Cleaned corrosion from connectors - problem remains
4. Disconnected the ignition switch and bypassed it - engine works fine!
So, our immediate thoughts were that the problem is with the ignition switch and it must be fixed. I'm in the process of ordering NOS ignition switch from one of the members here (my mechanic wanted to fix it by I simply don't trust the quality of the results...)
One thing which bothers me though... how come that the engine shuts down precisely at 6k rpm? I would expect it to happen at random values. This consistent behavior made me think it's a VBoost problem at the first time...
Thanks a lot for your help. I will update you when I get the new ignition switch installed.
Adi