Missing Boost, 90 200q

[Trevor Topley]

Thought I would give the collective wisdom one more try, my 200 is again
back to 1.7bar max, and at high rpm that drops off to 1.5b, I'm sure it
is caused by something silly since the condition comes and goes.
When the boost is "controlled" it can be set for say 1.9b, gets there
very quick and will stay there to 6000rpm.
Now it only gets to 1.7b, even with the tube to the wg clamped off!
There are no noises, no driveability problems, idle is fine etc.
I wondered about the controlled purge cycle and tried clamping off the
other hoses, but could not locate anything that effected the problem.
Has happened 4 times or so now in last 3 months, ie it has returned to
1.9bar without me knowingly doing anything.
As  it is now there is still a noticeable rush in 2nd gear, but in third
its seems quicker to upshift to 4th arond 4500.
I expect the motor will last longer as it is now, but will happily trade
that risk for the higher boost.
A reminder, my boost controller is manual,WOT switches etc not involved,
will go to 1.9bar at 1/4 throttle!
Any ideas???
        Trevor Topley