Oil pressure

I have a 88 scottsdale with a 4.3 and when I break and sitting at a light or wherever the oil pressure drops to 0 and the check gauges light comes on. It always seemed to be kind of messed up it used to flux alot for no reason. There is oil in the truck. What exactly is the problem?

I would check for a faulty sending unit first. Mine did the same thing I just added a mechanical gauge.