after having a head rebuild, on my s3 4.2, I'm experiencing some new issues. whilst driving along, the engine stalls. when I try to start the car straight away, it won't fire. however if I let it sit for 5 minutes, it starts no problem.
what have I bloody done?

 

The list could be long.

My ideas are:

Loss of spark, as in ign module (front of inlet manifold) is getting hot, dying, shuts down, cools a tad, away you go.

Fuel pump is stopping. Relay?, or switch contacts inside the airflow meter?.

Some earth straps have been left off when the head was refitted.

Basically some diagnosis, is it losing spark or fuel?.

 

I spoke to my local mechanic today who has worked on a few jags in the area. he basically said the same things as you just mentioned.
seems to be spark. the car car doesn't cough and splutter like its out of fuel. it just turns off.

 

OK.

Ignition col is another hot contender. They are getting OLD now, and are in a dumb location, so get very hot and that will cause an instant shutdown, then it cools, then you go some more.

It is strongly related to the ignition amp unit. I usually fit a new module and coil as a "cover the bases" method.

The engine earth strap/s are the cause of strange shutdowns sometimes. They should be checked and even removed and cleaned, just to be sure. If it does not fix this issue, it will certainly do no harm.