Well, I've successfully completed one of my patterns. I wanted to let you all know what changes I implemented:
1) I moved the vac and router AC supply cables from anywhere near the Z-axis motor cable.
2) I went back to my standard 3 flute carbide cutter (rather than the 4 flute cobalt I'd been experimenting with)
3) I lowered the speed of the cutter from full out, to 22,500 rpm at 20 ipm.
4) I periodically monitored the progress of the cut.
One successful cut does not mean I have solved the intermittent problem, but there is progress!
If I get 3 in a row, I'll post once again just to let you guys know at least one of the above changes was effective.
And a big thank you to all who offered their insights!!!
Chet