Change the oil filter again and see if that helps.
Sure sounds like a blocked or partially blocked oil filter hence the fluctuating oil pressure when the pump starts building pressure.
What you would normally see is a low pressure when the engine rpm drops to idle and then a rise to normal pressure as the rpm increase but never a jumping around of the pressure at a steady rpm.
Before changing the gauge try cleaning the oil line to gauge provided your set up has that type oil gauge and I think it does.
To clean that line take it loose at the gauge and sending unit with compressed air blow through the line until it is clear.
If you have an electric sending unit remove it and clean the sensor end. In either gauge configuration as Stuart says "let it purge for a few seconds" before reinstalling everything.
