1998 Renault Megane

A warning light has come on, its a orange squar with a zig zag line running across it which is now permanently on, the car will not tick over at a standstill and cuts out at very low speeds, especially when cold, although it ticks over when warm the rev counter keeps going up and down between zero and 1000 revs, but other times its ticks over rapidly at up to 2500 revs, is it a part I can get from a scrap yard and do myself
thanks for any help