Boost gauge flutters when dump cap is removed

["Turbo-T"]

Never seen this happen before...it's not ever done it with the cap on, just the cap off. Off the turbo is a THDP 3 inch into a 3 inch straught shot exhaust into a Dynomax Ultraflow. The wastegate is an internal design.

 

[WarWagon]

When I open up my dump/cutout I'll gain 2-3 psi and get a small boost in spool up speed. I have a 3"DP and 3" exhaust like you but differ in muffler type, external gate and I made the DP.
I'm thinking this is what's happening for you too. Back the boost down 2-3 psi next time you open the dump and see if the gauge still does it's thing.