As you can see, there seems to be signs of flash over / arcing on the number one cylinder spark plug.
After a while the problem seemed to come back, and I couldn't understand why. Then while I was doing my front oil seal, timing belt and water pump I had to pull the replacement plugs:
As can be seen, there seems to be arcing on the number one cylinder spark plug again! ( in addition to what looks like carbon fouling on the number 1 & 4 plugs, but that's a separate issue )
Any ideas what the heck might be going on?
EDIT Nov. 13th 2016
So @FredWilson was right about the boot having a carbon trail in it:
However I couldn't see anything like that in the #4 plug boot. I think I'll try the dielectric grease by itself first just for fun to see what happens.
POSTSCRIPT Nov. 28th 2016
So I pulled out all the plugs, and cleaned the porcelain sections and the plug boots with IPA and 320 grit sand paper to remove the carbon trails, then I slathered some 3M Silicone Paste ( dielectric grease ) on the porcelain part of the plugs and dabbed a little into the boots.
So far so good, the hesitation is gone and the increase in power is noticeable. It seems to have helped somewhat with the idle as well, although that's really a separate and intermittent problem, which I suspect is due to either clogged / leaky injectors or a wonky fuel pressure regulator.