2011 Ford Flex, standard V-6 and typical equipment, and 9000 miles with no problems, suddenly won't start one morning. No buzzer, no lamps on the instrument panel, nothing; just like the battery had been removed. Jump it off and it took about 20 minutes before it finally cranked and started. It did this twice, so we jumped it off and took it to the Ford dealer. They could not find anything wrong with it, so we went and got it. So far, it has not done this again in three days. Advice?