Gauge is a relatively cheap non-fluid Mr. Gasket mounted to the hard lines coming off the carb bowls. I have a vacuum/fuel pressure test gage that I can plumb in to verify if needed, but I think this happens with the hood open if left to idle for a while.

I'll check the vent line. I thought I blew it out. I did have an issue with the check valve at the 4:1 connection for the vents from the tank. Easy enough to pop the gas cap off. It's older than hell and supposed to be non-vented.

So a vent problem would cause the pressure drop?

Normal to see the bounce? Will the regulator smooth that out? i.e worth the effort to install it?