Wastegate, how does it work?

[Smeins, Larry]

My 87 5kcst has been suffering from low boost for a long time.  I've gone
over the boost trouble shooting items on the web page addressing boost to no
avail.  I even spent several hundred at the dealer and the best they could
do was get the boost to 1.2 bar at 6000 RPM and that improvement soon went
away.  Even with low boost, presently 1 bar at 5000 ft altitude, the car
runs fine so I have been living with it.  I've always been suspicious of the
wastegate or its control but haven't dug into that item, yet.  A couple
weeks ago, on a very cold morning, I think the wastegate stuck shut.  I left
my drive and as I pulled into traffic I gave it about 3/4 throttle and to my
amazement I felt my self being pressed back into the seat like I'd never
experienced before in my Audi.  I looked at the instrument panel boost
display and saw 1.6 bar at which point the engine cut out a bit and recaught
again at 1.5 bar.  I think it was hitting the overpressure cut out.  A few
miles down the road I turned a corner and when I pressed the accelerator in
my normal fashion the front tires broke loose.  Wow, I guess I'd forgotten
what these 5k turbos could do.  After the car fully warmed up the boost
dropped back down to 1 bar max and is staying there again.  I think its time
to look into the wastegate, so, some questions.

I believe the wastegate is controlled by a combination of mechanical spring,
manifold vacuum/pressure, and pulse width modulated electrical.  How do
these inter-relate?

If I remove the vacuum line what will happen to the wastegate operation.
Will it default open, close and become purely mechanical, or something else.

The same question for the electrical control. 

Is there some way to block the wastegate closed to verify that my boost is
being limited by wastegate opening?

Any other trouble shooting tips?

Thanks for any help you can offer.

Larry