Ignition - Possible pre-ignition/pinking cause 

Something that came to light during a conversation with a colleague and friend may be worth passing along. It's one of those things that, once ingested, lurks in the dark corners of the mind until a pertinent situation comes along. Such a situation reared its head a couple of weeks back - so now's the time to spread the word before it disappears back into obscurity. 
Engine's that run on or pink (detonate) for some seemingly illusive reason may well be suffering from a problem that causes an apparently correctly chosen heat range spark plug to run too hot. The problem is worn spark plug threads in the head. This reduces the heat-path required to cool the plug, consequently it over-heats and causes the aforementioned problems. So if you come across this one, check the plugs for fit in their threads. If they're loose, try the next heat range up!