spark plug heat range questions.

I guess i could have kept it a little more simple and asked is it possible that an engine with high initial timing settings in high elavation running 91 octane premium fuel could greatly benefit from a step or two colder spark plug? I am really considering buying the autolite plain copper core 3923 plugs wich is 1 step colder then the current plugs i am running.
I'm thinking the thudding out the exhaust that i was hearing while my engine was at an idle was caused by the spark plug heat range being too hot and causing pre ignition while at an idle. but i guess i could be wrong....