Hey, just a thought here I am not certain if this would work but we used to occasionally insert restriction washers or restrictor plates in the location the thermostat normally would reside in something like these here.
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Perhaps playing with one of those maybe
When I was on the phone with the guy at Rodney Red, he had mentioned something about having to use those things on race cars that had similar issues , but he'd not yet run across a truck with the problem