Tried the boost gauge teed into a different line, the fluttering has stopped.

The gauge was teed into the barb that originates at the carbon canister and goes through a solenoid (red arrow), and has now been teed into the 6mm vacuum hose that goes to the bypass valve (green arrow). With this set up, all seems well.

I only noticed this happening after the engine was out to replace the head gaskets.
I must have initially had the gauge teed into the bypass valve line, then switched it over to the barb next to it when I replaced most of the vacuum lines.



Will swap out the SAAS gauge with the Innovate MTX-D.