The factory gauge is an on/off switch at 6 psi, it doesn't tell you anything except that you have SOME oil pressure. If its flaking out it could be the gauge or wiring or the sender. Best way to tell if you are having issues with the oiling system is to install a legit aftermarket gauge.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.