Had a similar problem with my '05 Safari Simba with the 5500 Onan. Would run for awhile then stop....30 mins or so off and would restart again. I didn't think about it until we got back from a trip....the shut off coincided with either climbing or descending a long grade. I had a low oil level. Not a lot but apparently enough to trip the low oil sensor and shut it off. Filled the oil back to full and not a problem since.