Car: 2001 Audi A6 2.8L Quattro

Issue: Intermitent starting issue

Problem: Car starts in the begining of the day, but will not start again unless I wait 1 or 2 hours. It cranks but the engine will not start. I have confirmed voltage, checked & changed the battery, checked the alternator and the charging is fine. The air filter was changed today (and I did an oil change today - which doesn't affect this issue).

What could be wrong? If you are a Audi A6 troubleshooting expert, please let me know your thoughts.

(Also, if you are in the south Bay Area (Silicon Valley), let me know if there are real Audi technicians who I can depend on with reasonable pricing.)

Thanks, Rich