Mechanical pressure gauges are, in general, far more reliable than electronic ones with cheap senders. On a race car an oil pressure sender for a digital dash would be mounted away from the heat and vibration of the engine, on a piece of braided stainless -3 hose. It's probably the sender or wiring playing in your case. All the critical pressure gauges on my car are mechanical (boost, oil pressure, fuel pressure). Unless you buy very expensive Aerospace type pressure transducers (senders) that come from the likes of Honeywell, they give trouble, especially bolted direct to the engine. far cheaper to buy quality mechanical pressure gauges.