Hi This happened to me a short time ago.

You probably (90%) have a vacuum leak. Meaning that the hose that goes to the vacuum gauge from the engine has come off its connector or some other hose has.
Remove the air cleaner and get out your CSI Flashlight and examine every one of those 25 year old hoses and connectors. their are lots of them.

Age and vibration cause them to come loose. You will find it. Start at the firewall drivers side and take your time.

Good Luck, J