Pretty common when the compressor side oil seal starts leaking and letting oil get past. Oil then gets blown into the wastegate actuator from the vacume port on the compressor cover.
The solution to this problem is to have the turbo rebuilt and replace the vacume lines and actuator. With new parts, you should be ok to re-plumb the lines back to factory hook up orientation. If not, then you can plumb your vacume line from the intake, but remember, if your turbo has been bad for awhile (IE: leaking oil past the oil seals) and you have a stock location intercooler, it's probably full of oil too. Oil from the intercooler will be blowing into your motor as well. You entire upper end could be coated with a fine coating of oil. This includes, upper plenum, throttle body, all vacume lines, egr valve, vacume distribution block on the top of the throttle body and intake manifold. If this is the case, remove the intercooler and clean it out, throttle body and upper plenum and clean it, the vacume distibution block and intake manifold. The other solution is to do the GM Top Engine cleaner kit, available at your GM Parts Dept.
Hope this helps.
Patrick