The oil pressure jump when you let off the throttle indicates excessive clearance in the main bearings. Here is what's happening.

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.

Try another gauge and another oil pump to be sure but if the problem still exists check you main bearings.