2000 V6 4-Motion VW Passat- Intermittent Starting

I have a 2000 VW V6 4-Motion which has an intermittent starting problem. The electrical system has been tested and no problems have been found. The starter and ignition switch have been replaced along with a temperature sensor. the problem seems to be worse in the summer months. I have been to the dealer, a VW specialty repair shop and Sullivan tire (who is typically the best option). No one seems to be able to diagnose the problem correctly. Of course, when it is at the dealer it usually starts right up. Also, the first staring effort in the AM is typically without a problem. The car is kept in a garage.

When this “intermittent starting problem” occurs, what, exactly happens?

“Does not start” is a very vague description. Please provide as much detail as possible.