Lately when the engine has warmed up, it can't idle properly - if I let it idle it cuts out. I can start it back up again but have to keep my foot on the gas to keep it running.

After waiting 20 minutes or so it seems to work OK again.

It's a 1998 2.8 V6. Also, a few weeks ago it gave the opposite problem, where for a short time it was idling really high, around 3000rpm. This only happened once, for about 10 minutes.

Any ideas what I can do to fix it? I used a code reader to read the fault codes but all it found was related to the secondary air pump and the brake light signal, which I didn't think would be related to this issue.

Any help greatly appreciated!

 

I had this happen to my 2007 A3. Turned out to be the "low fuel pressure" sensor. Had it replaced and fixed.

 

try the MAF first

 

My first look would be the intake system integrity, be sure there are no leaks between the MAF and the throttle body. This will cause a lean condition that creates a rough, thready idle. The secondary air system could be leaking too, check it out also.