My 2001 A4 has infrequent hard starting, and has been going on, very infrequently, since December, 2012:

https://www.audiforums.com/forum/b5-...wheeze-190345/

I later figured out that the common denominator is that the car has been sitting for days. But, not necessarily will sitting for days cause the problem. It sat for a week around Christmas and started right up.

I also like the coolant temperature sensor idea, CTS, as explained here:

https://www.audiforums.com/forum/b5-...-start-197990/

But, if the CTS is a problem, would it not happen whenever the car is cold, that is, most mornings? Why only when it has been sitting for days (but not each time it sits for days).

As has been pointed out, the CTS is cheap and easy to change so I could try that. But, any ideas on why only when it was sitting for days?

Many thanks,

John