I had this problem on my 8v. It was "hunting", going from 500rpm to 900rpm and back again, all the time, while idling. I've now sorted it by following these instructions:
1. ignition off
2. disconnect breather hose from pressure valve (big round thing) on rocker cover
3. plug breather hose with something
4. start engine
5. allow to idle for 1 minute
6. disconnect engine collant sensor multi plug (rear sensor on housing at front of cyl head, as in pic)
IF engine cuts out then reconnect the multiplug before restarting.
7. increase engine speed 3 three times to over 3000 rpm
8. allow to idle
9. adjust co if ness
