I suspect that water is slowly leaking from the cooling-system thermostat housing (dome-shaped and situated slightly to the left of the plugs, as you look at the engine from the front) onto your plugs. I recently had this fault. The best thing to do is to buy a new thermostat and housing gasket (gasket is often bundled in with thermostat). Target price is £5 (plus some gasket sealant, various sizes and prices).
Then undo the two fixing bolts, remove the housing-cover, remove the old thermostat (water will leak-out for a short while), shove a piece of lint-free cloth into the hole to trap debris, carefully remove the old paper-gasket and clean-up the mating surfaces of the housing and the cover, carefully remove the cloth and debris, put some gasket sealant carefully onto each mating surface, fit the new thermostat, re-fit the cover to the housing with the gasket between them, carefully refit the two fixing-bolts and tighten them carefully until they have gone all the way home and then just a bit more (do not overtighten), check for leaks (there should not be any at all), start the engine and let it run for 15 minutes or so to warm-up (check for leaks again), switch off engine , let it cool-down, fill-up cooling-system reservoir if required (on top left at back of engine, as viewed from the front). Hopefully no more spark-plug trouble but do put some silicon-grease (or engine oil or car grease or something) on the threads of all four plugs to stop them rusting-up.
Tom Derby, July 2008
