Problem solved! Thank you guys for all your input.

I took it to our local engine guru who ran some of "his" fuel through it without any sign of overheating. - 320º. His blend is 10% nitro and 20% oil including some castor. Engine turns 12,200 rpm with a 12x6 prop.

With my Cool Power 15% nitro and unknown oil % NO castor it almost immediately went up to over 420º! Problem solved!

I bought a jug of fuel at LHS 5% nitro and 20% oil including castor. I haven't run this yet but expect much the same as the guru's fuel. He said to mix it half and half to get the nitro up to 10% and get the castor in the mix. I'll try that too as I have an almost full jug of the 15% stuff.

Thanks again for your input.
Phil