Why is my oil pressure light coming on in my 2002 Audi A4?

I have changed the oil on this car every 3000 miles, use a Mann oem oil filter. The car runs and sounds great. The light comes on and turns off occasionally . Would love some ideas. It's a 2002 Audi A4 Quattro 1.8 turbo. Does the switch just go bad? And what does it cost to put in a new oil pressure switch? Would love to hear any suggestions.