oil pressure at low speed reads 120 increase revs oil pressure drops to zero
I assume you’re reading this oil pressure from the gauge on the dashboard.
Get a mechanical oil pressure gauge, and test the oil pressure. Could just be a fluke, or your dash gauge could be going out. Or, your oil pump could be on the way out, but oil pumps don’t often go bad.
Some GM gauges will read full scale (120 PSI) when there is an open circuit, replace the oil pressure sending unit.