Why is my fuel pressure gauge shacking?

Do you have a way to temporarily isolate that gauge from where it's mounted - as a test? Engine harmonics (vibration) could be at play - especially if it's not liquid filled.

Just went through this with my blower motor. Before I installed my hood I had an Autometer fuel pressure gauge mounted at the back of the blower where I could see. It was steady at idle and even cruising until you hit 45 mph or so. Then it would start fluttering just like yours. I wondered if it was a fuel pump issue too.

Well, I got on the gas one day entering the highway and it fluttered so violently it took out the gauge. I talked to a service tech at Autometer and he was sure it was engine harmonics that did it. When I finished my hood and moved the gauges from the back of the motor to the hood, the problem was solved.