HDi 90 loses turbo boost intermittently

[russ2k]

I'm experiencing a problem with my 306 HDI, at first I thought it was 'one
of those things' but its getting worse. It's losing it's turbo boost
intermittently when accelerating - generally it occurs around 2,200rpm and
goes dead for 1-3 seconds, then comes back and performs how you would
expect. Putting your foot down doesn't give any boost either during these
'dead' periods.
First noticed it on the motorway, got a sudden feeling of lack of power for
a second or two (around 3,000rpm) and then back to normal.
It was quite bad tonight overall hence my post, I also noticed when I pulled
into the driveway that the engine did not return to idle correctly
immediately (ie. 900rpm or thereabouts), it was around the 1,250rpm mark???
Just for a bit of background, I have always had problems with the car and
the ECU reporting errors - always related to the throttle sensors I think -
2 dealers have attempted to fix but its never really been right - never
experienced the loss of power though. They have changed various parts, I
cannot find the invoices but always related to throttle potentiometer,
accelerator sensors (probably all the same thing for all I know..!). They
have also changed the earthing on the said component and altered the wiring
configuration but to no avail, the ECU light still keeps coming on. Perhaps
this could all be related? My feeling is that there is an underlying problem
which causes these components to fault, when they aren't actually faulty?
Any info would be appreciated, at least i'll be armed with something!
Cheers