This thread should be helpful; I would scan the car for trouble codes and remove the oil pressure switch and hook up a oil pressure gauge to confirm the oil pressure is ok.
http://newbeetle.org/forums/1-8-liter-turbo/60604-intermittent-oil-pressure-light.html
http://newbeetle.org/forums/1-8-liter-turbo/60604-intermittent-oil-pressure-light.html