Your comment "I had the head rebuilt in March (many minor cracks welded)" raises some suspicions about one possible cause of the problems. Welding cracks in alloy heads properly is almost an art form. Coolant leaking into the combustion chamber from a crack in the head will promote misfiring, and can wreck spark plugs in no time.

Is it using coolant? Check whether any of the spark plugs look too clean (like they've been steam cleaned), or are wet. Remove the radiator expansion tank cap and start the engine then check if there is a too rapid pressure build-up in the cooling system by holding your hand over the filler hole on the tank for a few seconds then quickly taking it away. Does the coolant in the tank appear to have air or smoke bubbling in it? Does the exhaust seem to produce an excessive amount of steam or moisture after the engine is started from cold? Any or all of the above symptoms points to a crack in the head - either a new one or one which has not been welded properly before.