Someone else, see "98 Accord LX dies/shuts off while driving.." posted a similar/identical problem. I think the inputs are ignition switch, ignition module or bad solder joint on main relay. The best cheap way to tell if it is the ignition switch - Turn the car on and then off while carefully observing the lights and behaviors on the gauge cluster when you turn it off. If, while driving at highway speeds, when it shuts off, you see the same behavior on the gauge cluster as when you turn the key off, then it is a sure sign of the ignition switch. There are YouTube videos by scannerdanner, and ericthecarguy that you can also watch for more ideas. And, again, review the other post.