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