i don't have a Novacon, but i had a similar problem when i in stalled a vfd on my cnc'ed RF-30. It would go into E-stop mode after 10-15 min when just the spindle was running.
Too much RF noise from the vfd. I increased the debounce settings. Helped a little. Then i changed the carrier wave fequincy from 8 kilohertz to 14 kilohertz. Problem went way along with the anoying buzz/humming noise.
did they set anything on the vfd?
I am using a teco fm 50-202-c
Hope this helps