2000 A4 - mechanic cant replicate problem??Wont start!

I have a 2000 Audi A4 that is in great condition, although that it wont start time to time. I turn the ignition and the car is turning over but it wont start. I have had all the wires checked, plugs and new battery. I have towed it down to the dealership twice and each time it gets to the dealership it starts right up. Go figure! I am wondering if anyone has had a problem like this or suggestions. Please help!

Could be a few thing:

1. If when you do get it to start does it run rough then ok?
A. If so replaces the temp switch.

2. It may be the immobiliser it being the receiver for the immobiliser being faulty - I think it is a fairly common failure on the A4

3. Try using one of the other keys as the transponder in that key could also be to blame.

Thanks for the reply, I appreciate it. For one, I know it is NOT the key that I am using because I have tried other keys and it still wont start.

Can you illaborate more on the immobilizer?