Check all the tubes for leaks using mild suction rather than positive pressure (don't want to blow a tube off a blind connection someplace back in the fuse) and without the instruments connected. A hand vacuum pump for bleeding brakes is a nice tool for this. It doesn't take much suction to find a leak which would be indicated on the gauge as a steady loss of vacuum.

While it's less likely to be the issue, check the routing of the tubes under the seat pan. If your weight (via the seat pan) is pressing on the tubes then that is also a possibility. If you're seeing some +/- Gs in cruise then that "might" fit the "wildly swinging" observation.

Good luck...be systematic in your approach and you'll find it.