Your era bus had a run on Charge air coolers that leaked pre-mature. You should be able to run across there in 5th gear at 1800 rpm and downshift to 4th if needed, 3rd gear should not be needed unless your towing 15k lbs.
Check the simple stuff but I would bet your sucking air somewhere, could be a slight tear in a turbo hose and doesn't reveal itself until extreme conditions. Also do the soap test on the CAC. Yours will either be next to the radiator or stacked on the front of the radiator. Take a spray bottle with a little soap and spray it around the edges where the welds are with the engine at idle, if you see bubbles your leaking.

That's a couple of things you can check.

Later
bv