I drove less than half a mile from home today and had to stop at some temporary traffic lights, at this point the engine revs were hunting between 1000 and 1500 revs at about a one second repetition rate for no apparent reason. I couldn't work out what was going on until i stopped and lifted the bonnet, only to find I hadn't replaced the oil cap when i gave it a little top up a few minutes before leaving home. Engine had splashed a little oil around the bay but otherwise no harm done, fortunately oil cap was still sitting on top of cam cover. Once replaced all returned to normal. I can only assume there's a sensor inside the engine which detects oil pressure at the top end and wasn't happy.

School boy error! lucky the cap was still there though!

 

The crank case fumes go back into the intake again so leaving the filler cap off lets excess air that is not metered enter the intake and makes the engine run lean which it then tries to compensate for, hence the erratic idle