Could be your oil pressure sending unit going bad.

 

After my cam swap I has oil pressure but it wouldn't climb like it should when I revved it and a few lifters started to make some noise. I later found out replacing the o ring on the oil pick up tube was a must if you remove it. I ordered a red one and it fixed my problem. ( also when I replaced it I realized I had tore the original one)

 

Well, late reply here..

I had planned to replace the sending unit, but ended replacing my oil filter (even though it was brand new) and that seemed to solve the issue. The pressure gauge needle now doesn't go up and down a few PSI at a time, but rather stays static in the correct place throughout the RPM band.

Btw, it was a Bosch filter. I will now run Wix