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When under load or a steady cruise the crankshaft is pushed down against the lower main bearing shells allowing oil to escape around the grooved upper main bearing shells causing the oil pressure to drop. When you coast or let off the throttle manifold vacuum increases and the pistons pull the crankshaft upward into the upper main bearing shells reducing the area the oil has to escape and the oil pressure goes up. The hotter the oil gets the worse this becomes.



How can the vacuum pull the crank up? Vacuum is, at most, 1 atmosphere of pressure, while your compression stroke is, at least, 8 times that and is pushing down (even coasting). I could see if you had huge clearances, loose bearing retainer bolts, bent crank, etc that there is a possibility that the crank bends down to cap under load and then comes back up when the load is released, but even this usually won't give that much of a presure difference. Depending on the tranmission type and design, you could also get the same type of side loads put on the rear main when you change loading direction, but again that is a pretty big spike. You could even be looking at a thrust clearance issue.

I would look at the pump and drive, relief valve etc after confirming with a new gauge, as was said. If that doesn't do it, something is terribly wrong in the engine itself.