1999 audi a4. First start of the day it will fire right up, this has never given me a problem. However, after the car has been running for a while starting it up again is hit and miss. Sometimes it wi turn for about 5-10 seconds and fire. Other times it will turn for up to 20 seconds and not fire at all. Some times I can give it a few five second turns and eventually it will fire, other times this will not do. The old fashioned, rolling start, pop the clutch trick also works every time. I'm guessing it has to be some sort of sesnor since this is the only time it has trouble, getting fuel, spark , or air. 
Edit: Problem solved. Had to replace the coolant temperature sensor
Edit: Problem solved. Had to replace the coolant temperature sensor