Vectra 1.8 Idling problem?

I had the potentiometer renewed about 2 years ago and has been fine until about 4 weeks ago. Now it is extremely irratic on idle. Almost to the point it wants to cut out. Sometimes can be difficult to start. I have changed the plugs and air filter. Has not fixed the problem, its not just annoying but dangerous as when the revs go really low I lose power steering and ABS. I cant see how I would need a new potentiometer after only two years?? And putting it on Tech 2 is a waste of money as it will only tell you what area the fault is, not pin point it exactly.