Glad you solved the problem - the 3.3V rail is the only PSU voltage that is used directly by the mobo without local regulation and refiltering to a lower voltage. Your local power utility will put a recorder on your outlet so you can see what is happening over a day or two (some will let you record for longer depending on how many calls they have for the service and how many recorders they have). Be sure to run all your appliances (furnace, heat pump, Air Cond, dishwasher, clothes washer, clothes dryer, etc. thru a full cycle while recording to see what happens.
There is also a device called the P3 Kill A Watt. You can test the outlet voltage, current, instantaneous power, kW hours over whatever time span you choose, power factor and frequency. But it does not record, so you will need the recorder service mentioned above to catch Mins and Maxes, spikes per period of time, etc. Or you could observe the KAW during appliance usages to see what happens. They are usually under $30. around the web. The same thing (the same PCB in slightly different packaging) is sold under the Seasonic Power Angel name.
For my own use, I only buy PSUs with active PFC and full range voltage compensation as they are relatively immune to line side garbage w/o any extra equipment. It's worth the extra cost to me.
If you decide you need some help on the AC line side, consider getting a decent UPS as the good ones include voltage compensation over a fairly wide range without having to switch to the battery, which is there in case the voltage goes away entirely.
.bh.