Our VT Senator used to do that. When i took it to a mechanic, he put his diagnostic machine on it and nothing came up. luckily for us, he said he's come across this problem before, and it was as simple as tightening the connections on the battery tterminals. Even thou minutely loose, that slight lost contact will do that to a car.
He tightened it up by maybe half a turn if lucky, and haven't had the problem since. Hasn't stalled, or gone to conk out or anything. Maybe try tightening the terminals a bit more?
Follow Your Dreams......RIP Brocky 1945 - 2006......May God Rest Your Soul