I had a check valve in the oil pump get stuck open when a piece of junk got underneath it.
When I first fired the engine up cold, the oil pressure would rise very slow, but steady up to about 60 PSI. As the engine got warm, the oil pressure would begin to drop closer to 50PSI. The pressure would also waiver, kinda like yours. I knew I had a problem, but just did not want to pull the engine until oil pressure quit on me. So I added heavier oil - which raised pressure, but it still had issues. The pressure began to sink again, so in when some nice thick STP. Pressure went up a little again and was tolerable. Pressure was OK with RPM's and would only hit 40 PSI, but not good at idle, so up went my idle speed to 1,200 RPM ( it was a manual car) and it would hit about 12 PSI.
Finally after 7 years of this, one day I fired it up and the oil pressure was 0. My excuse to do a fresh rebuild on a 455CI I have. Pulled down the 400CI to find the a piece of trash stuck under the check ball in the oil pump so it would not close, and all the bearings were wiped out showing copper. Pics of the rod bearings are below and the mains didn't look healthy. Just a little extra clearances for high RPM racing, right?
Like suggested, change the gauge out first before assuming any big problems.
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