I have a fiesta rst and when i stop in traffic my tickover somertimes moves between 1300 and 900 rpm any ideas. I know it should be around 1000 with a warm engine it sometimes goes right down to about 700. Would it have anything to do with me running right out of petrol at the weekend! ( bit of shite sucked up from bottom of tank!!) any ideas gratefully recieved!

Gav.