Turbo problems, Suburban k2500 - 94

Hi, there! I've lately experienced some problems with the turbo on my 6.5 L diesel-engine. My observations:

1) black exhaust, bad performance intermittantly 2) checking the lever on the right-hand-side of the turbo: it's fully back with no vacuum when this problem occurs 3) when the problem is gone, there is vaccum to this lever, and it is fully to the front (engine running idle) 4) reading out the error-codes gives me 78 (Turbo Wastegate Solenoid Fault) 5) the vacuumpump is making vacuum even when the fault is there

I'm very suspicious to the solenoid placed just left of the diesel-filter, and assume that's the same solenoid as reffered to by the errorcode. But taking out this solenoid and testing it manually has not proved any errors. (Testing in a cool environment, problem has only occured with hot engine...)

Q: Anyone who has experience with this problem? Is it most likely the solenoid that is faulty, or can it be other things going wrong here?

Thanks a lot i advance, from a nice, warm summerday in Norway (yes, it's possible here too...)

Roald ( snipped-for-privacy@hotmail.com)