Put a mechanical gauge on there to be sure. The stock electrical gauge is slow in accurate as all heck.
If the gauge is reading correctly, though, there could be a restriction to the oil pump, like a chunk of something or a bunch of sludge clogging the oil pickup screen, perhaps...?
i had the same prob bout 4 months ago i actually posted a thread on this here aswell. eventually i just said forget it and the car was my DD until last week when the motor got pulled no probs whatsoever. but to be safe get u a new guage.
You either have something clogging the screen on your pickup tube, that pulls tight and restricts oil flow to the pump when it's sucking hard, or, your oil drain passages are inadequate to drain back the oil that's pumped at high rpms. Do you have a high volume oil pump by chance?
Mine did that too. Nothing to worry about. As long as it doesn't go down to 0 psi you'll be fine. Even with a mechanical it drops a bit. Seems like it should be normal to me.