Very occasionally (Once a couple of months) my idle revs wont hold as usual and fall to 500ish rpm almost stalling.
The last time it happened I drove to the petrol station fine, filled to the brim and set off again. I stopped at our lasses work and when i was sat there it did it.

The last times it happened I just switch off the engine as usual, when I get back in to drive again its fine??

Its a 51 coupe turbo, any ideas???