2001 Jeep Cherokee Oil gage drops down when idleling or dec

Engine Mechanical problem
2001 Jeep Cherokee V8 Four Wheel Drive Automatic

Once my engine warms up my oil gage goes down when I idle or press on my brakes. When I come to a full stop it drops to zero and my check gages light comes on. The gage goes back to 40 when I press on the accelerater. Today was the first time that it has done this in the 2 1/2 years I have owned it. I went and got my oil changed and was told that my oil was not the problem. Do you have any ideas of what the problem could be? I was told it might be the instrument panel cluster or the oil pressure sensor switch. Is it safe to continue driving the car?