Sounds like spark knock. Timing might be too far advanced or might have alot of carbon buildup on pistons. EGR might be clogged also. Pull a vaccum on the egr when engine is hot and idling, and see if the engine cuts off, if it does egr is good.
I have the same prob. I use premium fuel and the spark knock goes away. Ive tried seafoam, fuel injector cleaners, full tune up and all. Next thing im gonna try is the thermostat for the 22ret(10 degrees cooler than stock 22re thermostat), it'll help richen the fuel mixture a little. If that doesnt do it im just gonna let it ride. Cant justify opening up a motor that runs so damn good.