Engine Oil Pressure Warning Light On?

My Toyota CS Corolla sedan is an August 1988 build model AE82 with a 1.6 litre 4AC engine. Manual gear change. The mileage is 120,000. The oil light flickers when I'm idling in neutral despite there being plently of engine oil according to the dipstick check. When I place the engine in gear and drive on the light goes off. Could it be dirty terminal connections? Or could the oil pump be going. If so, can it be easily replaced by myself or will I need a professional? I can usually do some repair jobs myself. Regards from Sydney, Australia