Its that wanky dump valve mate

Ok, you say "the temperature was much much cooler than when it was setup.." ... outside ambient temps or car water temp?

Water temp sensor breakdown can cause misfires, but tends to be rough start up and crap warming up cycle.

did the rev counter jump around with the misfires, any flames out the back etc? Was it a "fuzzy" misfire, as in still firing on 2 or 3 cyls, or was it a hard "bang" with popping flames/ like hitting a brickwall miss?

Worth checking your quick disconnect multiplugs on the right side of the engine bay just behind the headlight. They corrode and get full of water and cause big misfires as the engine moves and wiggles the cables.

Any unusual smell of fuel?