if you aren't rubbing coilpack ignition , ie still on the dizzy - you will need to gap the plugs down to around 0.6mm.

the plugs ford will have supplied should be AGPR12PP8 - these are ok upto stg 1 ( 280 brake) and are very smooth at idle. but anything above that they run dangerously hot and have ben known to melt the tips resulting in lunched engine. i''d put some motorcraft 071C's in - they are cheap and run fine.

the missing for a coulpe of mins sounds ominous - could be a failed headgasket - or porous head - letting coolant leak into one of the cylinders when you switch it off.

if you whip the plugs out - one plug will be a lot cleaner than the others as it will have been steam cleaned for 2 mins each time you've started it if it is water ingress.