I disassembled my pump, cleaned it well, including the pressure relief bore/piston, and replaced the spring with a new one--there are aftermarket springs that are used to raise the pressure--you may have one of these installed??.
Also, old hot rod trick was to place a washer under the stock spring to raise oil pressure--engines in the "old days" ran with much looser bearing tolerances than modern engines. As you can see, fooling with the pressure relief valve, either intentionally are with a gummed piston results in pressure different than the engine was designed for. When the filter gets clogged with dirt, the relief valve opens, bypassing the filter. Filters clog up pretty fast! Also, the oil pump is a positive displacement pump--meaning pressure is directly proportional to engine speed, so when the engine reaches a predetermined RPM, the relief valve opens, again controlling pressure. Generally, a worn oil pump results in lower pressure--the opposite of your problem. If the relief valve is the culprit, replacing the oil pump will fix it; however, I'd try replacing the relief valve first. But I have lots of time.
