1999 Audi A4 Intermittent starting problem

My car starts fine when it's cold. However, periodically (every couple of weeks) after I've driven anywhere from 2 to 10 miles, stop the car, come back 5 to 15 minutes later and restart it, it won't start. The battery is strong, the engine cranks over fine, but simply doesn't fire. Finally, typically after 5 to 10 minutes and a dozen attempts, it will finally start. When it does start, it does so instantly, there's no sputtering, smoke, or anything unusual. The car acts like nothing was wrong at all. In general the engine runs well, has good power, etc. Also, when all this happens, there are no "check engine" messages.

When I've brought the car to my local shop, of course it starts fine so they don't have any idea what to do. I went to my nearest Audi dealer and they gave me a Technical Bulletin that suggests things including decarbonizing the engine, changing plugs (which I had done recently), replacing the fuel injectors, the thermostat, and the coolant temperature sensor. Some of these don't make sense and some I've done.

Do you have any idea how I can fix this?

Thanks.