After installing a new GTN 750 last summer, oil pressure gauge indication works fine for a about 30 min of flight, then drops to almost zero, no oil or cylinder head temp increase. After landing, checking, same symptom. Avionics shop checked pressure (OK) and grounding at gauges - Pressure test on ground are normal - After several short fligts, also found that if I turn Master OFF then ON, oil pressure signal is back. Any suggestions?