[SHI]

I've been getting the ubiquitous turbo limp-home mode on my D5, which I've almost fixed...

I removed each hose in turn and checked them, cleaned the EGR mixer (which was not too bad, 1/4 inch of gunk), new intercooler fitted, cleaned the turbo control solenoid (vac tester draws air easy enough through the gauze). No effect - until I blanked the vac line to the engine mounts and would you believe it the problem is solved - well almost...
A 275mile drive today and it went to limp home once...

During my checks I put the vac tester on the turbo control actuator. It moves up and down accordingly, holds the vac/position, but it looks like it sticks about half way. The fourth pump on the vac gun it barely moves, fifth pump it jumps on. Hard to tell how smooth the release is. I can hear a metallic sound as it moves, like it needs lubricating.
When driving the boost is not quite even, pulls like a train up to about 2500 then I notice a slight back-off of power, but still goes strong.
The time it sometimes (always until I blanked the mounts) trips up is at 3/4 throttle at 2500 rpm. This is associated with a rasping noise that sounds like a cross between Hanibal Lecter feeling hungry and a fan belt slipping.
With the bonnet up and someone revving the engine to 4k, you can see the boost rising as the hoses swell, but on lift off you get this suck and you can see the air box/filter, and suction hoses move/compress. You'd think the air filter was getting blocked, but its new. My theory is whilst revving, the EGR is open, displacing some of the air intake flow. When you lift off, the EGR closes, causing more air to be drawn through the air intake temporarily whilst the turbo spools down.
I wonder if the sticking vanes are causing temporary overboost drawing more air than the filter can deal with. When they jump to their correct position, normal power is resumed. On full throttle, the vanes position is further over, away from the sticking point, so the effect is reduced. I do wonder if it overboosts sometimes, there is some amount of pull for a moment. You can almost tease the engine on this sticking point, sometimes it sucks, sometimes it doesn't.
OR - is it the EGR sticking, messing up the boost/MAF ratio?

Brakes vac - I've noticed the car will not hold brake vac for long after switched off. Every other car I've had the vac holds for days. After an hour this has lost all brake vac. Surprise when you put it in neutral after parking in gear to supplement the useless handbrake. Curious as if the engine mounts vac can cause so much upset, is there a vac leak elsewhere in the system?