[JimC in NC]

I think I've seen this before and it was a cracked coil. When it got hot enough, the crack widened enough to cause the problem. As suggested, checking for spark when you experience the problem should confirm if it is an ignition/spark issue. Good luck.... no one likes to be stopped and troubleshooting problems on the side of the road, but that may be what it takes.
JimC in NC