On the 740's that was THE classic sign of a faulty fuel pump relay although I believe that was corrected with the 850's. Our old 740 would, as you describe, just cut off crusin' down the road. Evidently you have an ignition component that is getting hot and failing or more likely something like a bad ignition switch. Do you have a big heavy ring of keys swinging from the car key? Does it restart right away or do you have to sit awhile? You're right. It is pretty unsettling if it happens on the interstste.

bl