"when it ain't broke.... Replaced original plug wires. Reused old spark plug (3m new) - those expensive double spark... which I thoroughly cleaned with a wire brush (old habits never die but which I suspect may be causing my problem; must have overdone and damaged with agressive brushing which was not needed since the plugs seemed new-no deposits). Engine ran great but with a trace of rough idling which was the reason I decided to put new wires.

After replacement of the wires and old plugs the engine started with normal idle. About 5 seconds the engine surged just a little bit (no RPM gauge) with the 'service engine soon' on. About 1 minute later the engine idled normally and the 'service engine soon' disappeared. Whether I accelerated or not the light went off in 60 seconds. I drove all day with no problem. It only happened when I started the car the following morning. Remember this is summer. Could have different behaviour in winter time?

Before I replace the plugs does anyone have ideas about what might have causing this problem. Owned this car new for 15 years and never had 'engine service soon' light; I feel lucky.