Souds to me like the gauge isnt getting the correct information from the plenum.

In the best case, this points to a leak somewhere around the plenum, which explains the symptoms, as well as the boost gauge problem

Check really carefully around the plenum and inlet for leaks, and pipes that are off/ loose/ broken. With boost gauges, often the supplied plastic pipe can become brittle. Take it off and block it to test.

Just athough, but if this is 'T'd off a pipe that tells the ECU when to make boost etc, it may be defeating the boost on the wrong info. Check really carefully all the small pipes that go to the various bits on the inlet side, including the Map sensor (upside down on plate on inner wing), the Fuel Regulator (really important one, this one, can make engine go bang if loose!) and all others!

Could also be fucked gauge, but unlikely as you have other symptoms as well!

Let me know what you find

JJ