so my thought is its the CPU cooler thats failed causing the CPU to mass overheat on startup
Very unlikely, not a lot could cause a cpu cooler to break, and even if it did, then it's not going to cause boot looping, the closest to that would be the motherboard not turning on because it detects no cooler fan plugged into the cpu header, if that setting is on in the bios ofc, so I quess, maybe a fan that is failing and stuttering on and off might cause boot looping if that setting is on
if it worked when you bech tested it, then there are other, more likely causes for a bootloop, like there being a rogue extra standoff causing a short on the board, but again that is unlikely because of it not being a new build, did you bread board the old motherboard and components?
You've checked all the leads are plugged in nice and tight right, as it sounds like the pc might have been moved around quite a lot before you bringing it back out to use it again, yeah, that's far more likely.
The side of the case that is behind the motherboard never got a knock bending the face of it where the standoffs go in ?
you made sure that the old standoffs in the original case perfectly line up with the new motherboard?
the Last ditch attempt is getting a new PSU, and breadboarding that with the new, and old motherboard, don't go leaving CPU coolers off though, that's just lazy, and bad practice, don't want a definately broken cpu on your hands, do you? tut-tut! Don't break what you already have that is probably working fine, that's just silly, might as well just put it all in the bin if you are going to do things like that