Cooler plugs?

Cooler plugs will not help with a cooling system problem either. In fact, they may make it worse. I agree with the others comments, the engine is likely detonating due to carbon build up, which is made worse by the cold plugs. Get the carbon build-up cleaned out, put in new plugs of the correct heat range, and it should run fine again. I'd double check the timing as well.
By the way, engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the cylinder head and into the coolant. So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. It's a snowball effect. That's why an overheating engine wants to detonate and that's why engine detonation tends to cause overheating.