Power companies try to control load voltage in a very tight band. However as load is increased on the system the voltage tends to sag somewhat although it never goes too low. It is possible to locally (neighborhood area) to sag the voltage low enough to cause some issues. Your power conditioner should eliminate this slight fluctuation within reason. If the voltage fluctuates more than the conditioner can handle you should get some sort of light or beep. My APC does both and it locks in the light so I can tell it had a transient if I am not right there. Get a volt meter and check the voltage to the house when the problem occurs. Leave it connected and watch the value when the problem occurs. This might not be conclusive since the system operators are constantly striving to return the voltage to band. Bear in mind the same local fluctuation can occur inside your house if the power panel is insufficiently sized or you are running a large load demand (dryer plus oven/stove plus air conditioning/heating).