I would check for power issues -if it is a replacement KSU having the same problem. Check to see if they have added any new equipment on or around the same circuit as the phone system, ie. copier, microwave, Fridge. Check power at the outlet-If norstar systems fall below 117V they wig out, another problem I have run into many times is defective battery backup the way you test is unplug from Backup and to direct power to see if problem persists.
norstarguru