Good suggestion to check for RFI/EMI, though I would worry more about EMI/RFI if using a wireless connection than if using UTP
Ethernet - assuming the cables and connectors are in good repair, and not 100 meters in length. Since this is happening with multiple computers, it is not likely a cable problem, unless, it is a common cable - hence the suggestion to check the cable between router and modem, and the suggestion to have the link between the ISP and the modem checked by the ISP. Or if the affected systems are behind a switch - then swap the cable between the switch and router to see what happens.
And with this happening with multiple computers and operating systems, Safe Mode would not likely reveal anything either - and it likely rules out a 3rd party application too - unless, maybe, this same 3rd party app was recently installed (or updated) on ALL the affected computers. Worth checking, but don't hold your breath.
While it is possible a faulty microwave oven or other device that uses RF or high-speed switching devices is spewing out high energy RFI/EMI, I would think it would knock out the whole network at the same time, or those computers that are in close proximity would all disconnect at the same time.
If RFI/EMI is suspect, the best preventative measure you can take is to ensure the facility has a good station ground to Earth, and that each computer is connected to a properly wired, and grounded outlet. Every home and computer user should have access to a
AC Outlet Tester. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets too. These testers can be found for your type and voltage outlet, foreign or domestic, at most home improvement stores, or even the electrical department at Walmart.
Also, every peripheral directly connected to each computer should be powered through the same wall outlet to ensure they all share a "common ground". If for example, your computer's power supply is connected to the outlet on this wall, and your speakers are powered through the outlet on that wall, there WILL be a slight difference in the resistance to ground that could create a small "difference in potential" (voltage) between the two grounds and that can result in "electrical noise" that can interfere with other circuits if excessive. That said, I doubt that is the problem here, but I would check all the outlets in the facility - especially those powering the network appliances.
@Deek - was this network ever stable? Or did this problem suddenly start appearing? Any big changes in the office lately? I would not suspect a router exploit. Those are actually pretty rare and generally, they would be used by the badguy to hack your network to access information on your network, or to use that network to distribute malware, spam, or other nefarious deeds. In other words, the badguys would want your computers and network to keep running smoothly.
Have you reset your network?
