Well....I hate going in circles vs logical troublshooting, but that's what I'm doing.....
Bought a cheapie coil (available locally with no wait) just to try. Problem seemed gone, but just as I was heading home from a test drive, things started popping and snapping out the exhaust as soon as I started to accelerate. Sure seems heat related.
I have removed both valve covers looking to find a broken outer valve spring. Everything was OK.
I checked the plugs. Front 4 were 1/2 gray and 1/2 white on the porcelin. Back 4 were all gray on porcelin. Looked pretty normal, although I was using the 11Y vs the 9Y plugs.
I have twin Holley's and am going to check the vacuum secondaries next in case one is sticking or eratically operating.
Only other mechanical check is to drop
oil pan and check for stretched rod bolts (as the cause for the high rpm "tchunk"), but I kind of doubt that's it.
I'll see about new 9Y sparkplugs. Better heat range and the new ones in it look a little like high speed glazing.....only because they are turing gray vs tan.
.......running in a circle like a chicken with it's head cut off......not fun at all !!!