If the home has an attic space above the ceiling, you may very well need to go up in to inspect junction points as was already mentioned, but I don't know if you understood where the location of these splits would be.
If you want to figure this out without spending money then will require MANY troubleshooting steps because there are too many variables. Even if you eliminate the PSU and UPS as source of problem, then you still must inspect the breaker panel and the outlet, which it seems you have already done. After that, it's time for someone to inspect the junctions, assuming there are any. Junctions will be found in attic and hopefully not in-wall which would only happen if your parents hired a shady contractor. (yikes!). If everything about the circuit in question checks out (proper term, guage, etc) after visual inspection and no problems are found then the only remaining causes could be the PSU and/or UPS.
My gut tells me your PSU is not up to the task. My gut feeling has been wrong countless times though, so don't hold me to it.