@dc42 - Thanks for adding your thoughts. First (in case you are busy) - we replaced the cable to the X endstop switch and the symptoms have gone away. So we are back in business.
But to your question - yes, the tool heaters showing a temperature of 2000C was part of our collection of symptoms. In the end we were able to make the problem occur consistently by setting the end stop to be an M581 T0 trigger and then activating the switch. This caused a machine halt that and also included the 2000C display. We're assuming that the 2000C is a byproduct of the halt and not part of the failure mechanism.
We were also able to cause the halt by commenting out all of the tool configuration (made us happy that this is in a separate macro) for both tools. We still forced the halt as expected.