Holden Astra 2000: What is causing it to run rough?

RECENTLY I had the timing belt and adjusters replaced on my 2000 Astra thanks to your column. It had 100,000km on the clock and I didn't realise there was a problem until reading Carsguide. Recently it has been stalling when stopped, missing when driving along and periodically running rough. The engine-management light comes on and goes out soon after. My regular mechanic could not find the problem and rang his mate, who said there was a problem with some transmission computer programs and Holden was installing a new program. I took the car to my local Holden dealer who could not find the fault because the computer wasn't showing any problem and the engine-management light wasn't on. They cleaned the throttle body and some contacts, but the problem still occurs. What can I do?