Certainly no expert here, but out of curiosity, how do the plugs look? Are there any signs of overheating, what does the mixture ring look like, etc..

Point is, it sounds like you really don’t know what’s going on – as in, is this an issue of excessive heat production (incorrect ignition timing, mix too lean), or an issue of improper heat flow through and out of the engine (improper exhaust flow or weak cooling performance)?

If the plugs are still new or very nearly new, they might not be able to provide enough info. If that’s the case, consider putting in a good, used set and run those for a while, to get a better read.