Have you stayed in the room (with UV-safe glasses, of course) to see if the exposure completes (counts down all the way), or if it aborts at some point in time?
I have a different NuArc, so I can't comment directly, but you could also check your line voltage during exposure. It could be dropping too far and causing a problem for the electronics.
Anyone know if the 26-1ks has a thermal cutoff?
Lastly, M&R Print, which owns the NuArc brand, services these machines. You should contact their service department. They should be able to help you.