1996 GMC JIMMY 4.3L

1996 GMC Jimmy
REPLACED OIL PRESSURE SWITCH THURSDAY NIGHT. OLD ONE WENT BAD AND LOST ALL OIL. TEST DROVE AND HAD 50 PRESSURE AND NO LEAKS. YESTERDAY MORNING SAME THING. YESTERDAY AFTERNOON IT SAID 20 PRESSURE AND DOWN TO 0 WHEN STOPPING. STILL NO LEAKS AND DIPSTICK SAYS FULL. MOVED IT FROM DRIVEWAY TO GARAGE LAST NIGHT AND PRESSURE SAID 60 AGAIN AND AGAIN THIS MORNING WHEN I STARTED IT AND NO LEAKS. WHATS WRONG?