Most of such 'sensors' operate by being 'grounded' to the engine; inotherwords, if any oxidation or corrosion develops between the sensor and the engine you will get 'erratic' signals.
Rx: remove the sensor and clean/polish the surfaces of the press. sensor that come in direct contact with the engine .... ditto the engine contact surfaces (and also check/validate the main engine 'ground' connection for high resistance).

As andy posted ... its a good idea to 'tee-in' a redundant mechanical pressure gauge.