[urq] Wastegate diaphragm failure mode?

[Louis-Alain RICHARD]

Hi there,

My car is again possessed by the 4000-5000 RPM cutoff. In fact, it is very
intermittent and not always at the same RPM (but always at WOT).
More, the engine never felt so strong (yeah!) and the in-dash boost gauge
seems to indicate slightly more than 1.8 bar. I used to linked that to the
fact the outside temp is very cold (-25C tonight) so the boost is at its
max.
The connections at the air temp sensor are nice and clean.
The idle switch is not flaky, at least not when I checked it.

But I can hear a very small exhaust leak (or is it boost?) coming from the
passenger foot well area.

So my mind is wondering: 
if it was the WG diaphragm that is leaking boost, could I hear it?
If yes it is leaking, could this cause an over boost situation and then the
CPU would cut-off the fuel pump, explaining the different RPM experienced?
If yes, I supposed a Mighty-Vac would be the tool to find to test the
wastegate?
If yes, where can I find a diaphragm? 
How much does it cost? 
Do they fail often?

Thanks in advance,

Louis-Alain
83 urQ