Oil Pressure gauge freaking out

So I changed the gauge with one i knew was working. The psi stays steady until the car gets to operating temp (180) then starts to twitch and then flutter like crazy. I also tried my original gauge on another car and it worked fine. Before all of this eveb happened, I cleaned out the line.

So I am lost, cold start psi is steady untul operating temp then it flutters. Could it be the oil? It's 10w-40 zrod and the filter is wix.

What else could it be?