Is the oil pressure gauge "Direct Mounted" to the engine? If so take it off and shake it to see (hear) if it has shaken the internals apart.

We have had this problem several times on our VW drag motors due to vibration. Several gauges would not even last the weekend before erratic readings. The cure for us was to take the direct gauge (or sender) and place a short piece of -3AN stainless hose between the engine case and the direct gauge (or sender) to isolate it on the car body.

Also sounds like you have a relief valve problem. Are you using a filter with a valve?