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Pre-Ignition/Detonation Problem Maybe?

This may be a moot point, but if you are not running 91 octane gas, maybe try a tankful and see what happens.
As your engine accumulates miles you get a small carbon buildup on the pistons and inside the combustion chambers. This has the effect of reducing the squish volume a little, which increases the effective compression ratio.
This may be why you are noticing detonation now and not before.
You may be just on the fringe of detonation when she is hot, especially if you are running less than 91 octane.

Rod

There will be very little carbon build up at 5000 miles; certainly not enough to change squish. The OEM head gasket is .045" and the pistons in the later engines are typically down in the hole. There is plenty of room for cabon builid up before it will change compression.
Got to agree with Dolt here, it would take a lot of cheap gas to cause too much carbon to build to create the HOT spots that will cause flash over and detonation, a knick in the head or piston would change the compression and need for higher octane fuel more IMO of doing lots of valve work over the years
 
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