You could have a problem with the Turbo Boost Sensor (mounted on the firewall), or with the wiring. I suspect that the former owner might have had a fuel cut defeat installed, and this is either messing with the signal or if he removed it, the wiring may not have been properly reattached.
I will also suggest that you don't bother worrying about the stock gauge. It really tells you nothing, is slow to react, and shouldn't be used to diagnose anything on your car. Get yourself an aftermarket gauge - any kind is better than the stock one.