Couple of pointers first:
1. If your putting 42Vdc into the HCNC board it's won't last long in all probability.
2. Your heat sink needs som hole in it to allow airflow and reduce a thermal gradient.
As to your problem, reflow your solder joints for you power connector, voltage regulator and components nearby the voltage regulator. In all probability you have a cold solder joint.
Is your 5V ok when the 24V out is low?
Phil, Still too many interests, too many projects, and not enough time!!!!!!!!
Vist my websites - http://pminmo.com & http://millpcbs.com