There's a couple of things I'm curious about.
Are you saying there's a problem with more than 1 computer? If so, are they on the same ups? At the same time? Always?
Does the UPS sound an alarm or beep?
How many times has this happened? You said they are not on the same multiwire circuit? on I'm just asking these things to get as clear a picture as I can.
Is is a long run from the panel? Never hurts to check tightness of connections.
Any power fluctuation that would affect the computer should be sensed by the UPS first. There could be something wrong with it (ups). If you're talking about 1 computer there could be some problem with it not related to the Vac. Or, you could have a voltage drop problem (for various reasons) and a ups problem.
Just tossing out some ideas.