Volvo CX90 temporary loss of all electrical power

Car lost all electrical power while traveling on highway. The car goes completely dead, then after a few minutes, the electric power kicks back on. It rarely happened but is frightening when it occurs. First occurred about 9 months ago. Volvo said that there was nothing that they could find wrong. Then it started doing it about two months ago and Volvo said that there were lots of diagnostic error messages and it needed a new engine control module and reprograming of the codes. Car functioned find for about six weeks. It went totally dead while traveling at 60 miles an hour, came back on, and then cut out again, and again. When I say totally dead, I mean absolutely no electric power at all. The emergency flashers don’t even work when this happens. When the power returns after a few minutes, the wipers go on once and all electric systems return. Volvo has had the car for a week and driven it 100 miles trying to get it to repeat the problem without success. My wife and I are convinced that this problem will result in a serious accident if not fixed. Any thoughts would be appreciated.