I have never seen an oil pump stop working then start working again. Most of these oil pumps are gear driven off the front geartrain and are very reliable. Possible causes: low oil level, incorrect grade of oil, debris covering and uncovering pick-up screen, pick-up tube loose, sucking air on supply side of pump, worn out pump, worn out bearings, sticking oil pressure regulator, etc. Put a mechanical gauge in an oil galley and watch pressure hot and cold at various rpm's. Before this question was even asked, I would have changed oil and filter to evaluate. How many hours on this engine? How many hours since last oil change? Philip.