Check the hose connections off the intake plenum...right after the lower IC tube at the top/left side of the motor, after the ACT sensor. Follow it back towards the firewall as best you can to be sure it is intact, not split, etc.
Then you'll need to check the hose where it connects to the rear of the instrument cluster. Speak up if you've never had that apart and need hints, etc. If all the connections are ok, you might suspect the gauge, but they're fairly reliable, I think.