Oil Pressure

[tempest]

This happend to me the other day, i was driving into adelaide, from the freeway, all was fine on freeway, once in adelaide, i come to stop lights, sitting there waiting, and suddenly my oil pressure starts beeping at me, i pulled off, keeping eye on it, and it raised back up to were it normally sits, next stop light, same thing. pulled over, checked oil levels which was fine.wat ive determined, is taht wen under revs, the pressure is normal, however wen idling, it drops markedy. i dont know much bout mechanics of cars, any pointers/suggestions about y this suddenly started happening. cheers