Just came back from my 60K service at my dealer. I learned a couple of things about this RPM thing.
1) Isuzu put out a bulletin about the EGR valve getting stuck/clogged, with the result of the potential of burning oil to piston failure.
2) My Izusu mechanics told me that a particular wiring harness in the engine bay contains a wire that controls idle which has on several Isuzu's frayed or even shorted out. The wild fluctuation of and high RMP's were a symptom. THey showed me where the harness was located and said that if the symptoms of high or fluctuating RPMS's happens, to giggle the harness and see if that causes a change.
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