Easiest and likely cause is that the pressure hose on the cold side of
the turbo has come loose.  Check that ducting, should be held on with
a hose clamp which has been known to do that on occasion.

Might not be it, but easiest and cheapest possible fix.  Let us know
what you find one way or the other.  More than a "bang", was it a
"fwoosh" by any chance?

Dave Hinz

Could be.  But more likely that the apc (turbo control) solenoid is
sticking.  If it never sends vacuum to the wastegate the wastegate
won't open.

The jerking was when the boost pressure safety switch sensed that the
boost had gone too high. It cuts the fuel injection (and maybe spark
too, I forget)

-Fred W