Engine not idiling


My Charade 1.3 (1993) will not idle below 1500rpm. When I am stopped at traffic lights the engine rev's up and down between 1500rpm and 2000rpm, without me touching the pedal. Would there be a faulty sensor. I did have the heater radiator repaired earlier this year, is it possible that water may be in the box below the heater radiator. The above fault did happen before, but then engine rev's were ok. Hope somebody maybe able to help me. John Boyd