#1, there are different types of UPS power supplies. Not all smooth out the power variation (most don't). The type that do are expensive - basically they don't simply pass power; they charge up a battery in the UPS then invert the DC battery power to AC line voltage. These are called "double conversion" UPS units. Line interactive also do some level of power conditioning/leveling, but only at the extremes.
#2, if you are having "dirty" power, your utility may be able to put a monitor on it to see what's going on. It could be a poorly bonded connection in one of their boxes causing the problem. This is worth asking about especially because "dirty" power can damage other things - HVAC, refrigerator compressors, etc.
#3, it could also be a poorly designed power supply board in the machine itself. In this case, unless you can smooth out the power variation, you will always be fighting poor design.
#4, your house wiring design could be a problem. If things with motors are on the same circuit as the machine, they could cause noise or voltage droop/spike. Try not to have items with computer boards on the same circuit as a refrigerator or freezer or similar items (in newer construction, those items usually have dedicated circuits as required by code).
I considered getting a more expensive UPS (I think that would be one with sine wave output). We don't think we will be in the house permanently, so I haven't been willing spend that $$ yet. I did talk with someone at one of the companies that sells these (Tripp I think) - to get an idea of what I should purchase. But they said it would be hard to say as we don't know exactly what the problem is.
I hadn't thought of contacting the electric company. I had considered contacting an electrician. Worth asking about for sure!
Since the machine seems to work fine at the shop where I bought it (and it has had routine maintenance there 2x) - I'm thinking the power board in the machine is likely okay.
Our house has aluminum wiring. The connectors have been updated (alumicon connectors). However, we find the house is wired oddly. Upstairs, where my sewing room is, it seems like a lot is on one breaker. Definitely have lights slightly flickering here and there. I think the circuit board in the house should likely be replaced. We've replaced some wiring as we have done some home reno projects.
If I had room in the car on the next trip we take to see our kids (who live in a house newly constructed just a couple of years ago), I would take the machine with me and try it out extensively while there. Might have to see if I can finagle that. Would definitely rule in or out the problem being with the machine (I don't think it is the machine - but you never know!).