whatever is going on in the ecu, dropping the clutch to idle rpm would get the power back on mine.
when overboost happens, usually ECU is informed by MAP sensor.
There is pretty good explanation (with simplified formula) on wikipedia how MAP calculates pressure. If anyone is interested there it is.
When ECU detects overboost, first it decreases fuel pressure in rail. if it continues to rise, it kills injection completely. to prevent engine damage.
So, first you have power drop. if you continue, power is cut off completely.
in first stage, power is dropped and by pressing clutch decreasing of pressure occur because RPM falls down=> MAP continues to pump fuel.
also, ECU on JTD (from early models) have a feature:
when you release accelerator pedal and clutch is NOT pressed, it dramatically reduces amount of fuel injected. (that is why, among other things, it is important for clutch pedal to touch clutch sensor in released position)
some documentation say: completely cut off injection of fuel, almost ventilating combustion chamber with pure air in that moments.
I don't believe in this but who knows?
Overboost problem is actually simple to identify and solve.
turbo vanes are stucked in one position:
-either stucked vanes
-or vacuum all the time in pipe that goes in vacuum driven actuator mounted on turbo housing.
stucked vanes...pretty rare case but I once put link to this story, with all the pictures.
vacuum all the time?? how?
VNT solenoid does not allow air to come in pipe that leads to turbo.
so vanes in turbo are always in position for low rpm...which means, turbine spins easier and faster.
and goes like this until it reaches some point (usually 2500-3000rpm) where pressure is so big, it triggers MAP sensor alert.
EASIEST way to test:
when engine is off, remove small diameter rubber pipe that goes from VNT solenoid to turbo.
I usually remove pipe on side of VNT because it is easier to reach
if you hear sound of air sucked in that small pipe
that means that vacuum is present. it should not be if engine is switched off.
one more way:
do not perform previous procedure. just take with some tool rod that moves vanes inside turbo.
if you can not move FROM turbo, if it is already moved, then vacuum is present also.
if I put pictures it would be much more clear what I mean.
Jasons understand what am I talking about. he is better with words. he can try to explain