Nec model FE1250 problems.
 

Now I have two exact same models here, one my tech could not diagnose
heater going out after several hours of time but will appear.
Knocking, heating etc chilling components does not affect the problem.
Even tapping the CRT neck doesn't cause the problem either.

The latest one just came in few days ago and now my turn to try to
diagnose a "shut down" problem, this problem, according to customer,
extends the problem longer if cooled with fan on that monitor.

See if your NEC have bulletins on these model.

Thanks & cheers, Wizard