[mark houghton]

The erratic boost gauge is telling us something. Either a faulty boost sensor unit (hope not, they're about $150) but since it seems to be tied to your problem, it's almost pointing to the WG. But then, if your boost gauge is doing the chicken even when not on boost, then the sender or gauge or connections could be the simple culprit to that symptom...regardless of the other engine running symptoms. You may have two separate issue going on here.

So lets for now forget the funky gauge bouncing. Do pull the boost cut-out switch and put some external pressure to it. It should open and interrupt ground at around 1.1 to 1.2 bar I believe. If it opens much sooner than that (like way before .8 bar) then you've found your problem.

And do check the WG operation as already mentioned. Leave the top port vented to atmosphere, and connect a pressurized air source to the bottom port. Stuff a wad of paper in the WG exhaust pipe, and with the car running (don't forget to block the now open line to the intercooler) your wastgate should open at anywhere between 15 to 18 psi...and blow that wad of paper out. You'll hear the exhast note when it opens.