unexplained monitor shutdown

I am dealing with a small business that has 5 workstations. There is one server in the back of the office, one w/s in an office a bit further towards the front, and 3 w/s in the front (customer counter). 2 of the w/s up front are connected to the server via network, and one is a stand alone. At times, all 3 monitors at the front counter shut down at the same time. Computer stays running. All three are on UPS units. The only other thing I can think of is magnetic field interference. Any ideas on causes / possible fixes would be greatly appreciated! Thanks!