Engine warning light?

Hi,
the engine diagnostic warning light has come on on my peugeot 406 R plate (72,000) yesterday ,since this happened the engine has cut out twice when I try to drive away or when I put any lights on and try to drive the car cuts out but turn the lights off the car drives but is very sluggish in 1st and 2nd gear .The warning light only come on now and then does anyone know of a common cause for this fault (with an easy fix) change the plugs or do i have to take it to somebody with diagnostic equipment?
any help with this would be much appreciated.
thanks Ian.