No start with warm engine, round run, soaked!

Hello forum,
I haven't bothered anyone with stupid questions for a long time because my old V8 (3,6) behaved excellently. But that's the end since yesterday....

First, the fuel consumption (in every load state) has increased dramatically. After a short stop, the car only jumped back after about 15 minutes of cooling. Thereafter (except for the still frightening consumption values according to BC) no more problems, although some city trips were necessary.

This morning, as usual, a smooth start, then a ride through a larger city, a longer distance over a tiny highway and again a motorway, driving behaviour not optimal, but "restrictable", but again very high consumption (actually around 20 l).
After stop (about 200 km) no start possible - starter turned, but no ignition. ==References== Two hours of waiting, the car jumped again... and then he took bad gas from the lower speeds.
I immediately drove to Audi and without switching the engine off to the diagnostic level for error reading.... But that didn't work - even though the engine was running, an error message came. It was like, "No contact ignition, terminal 15"
So, halfway in time, no diagnosis was possible, I was advised to go home cautiously....
Unfortunately, I had to turn off the engine again. After that, the car did not start for three hours. The ADAC breakdown service sprung up several "start pilots" and organized the return transport after unsuccessful efforts. Just - when unloading at the local workshop - (after about 6 hours of cooling) the car started immediately again and runs (possibly only on 7 cylinders)...
What cause(s) is (are) likely?

I've already read that maybe the OT donor has to be exchanged, but would like to know more... also about possible cause of the problem "non-readable error memory".

Greetings and thanks in advance for "relevant notes".

BTW: A timing belt change is about to take place. Which company is recommended in the area Cologne-Bonn (or drumrum) for this or does anyone here have competence and interest in doing this work?


Wintho