I'm sorry. What was I thinking ?? You are right.
In our industrial application, all the high-draw and resistance applications (like say 2200 degree high vacuum furnaces and electrical welders) are on isolated fuse boxes, busbars, power cabinets and main lines all the way back to the base transformer station. We have a nominal electrical use in the area of a million dollars a month. Every system is certified to NFPA 70 (the national electrical code) to be no more than 70 % of the rated electical breaker box at full draw. Every system drawing more than 20 amps has a dedicated ground that is a minimum of 1/2 inch in diameter sunk three feet or more below concrete level. All of this, and there are still significant voltage variations. Current supply will usually remain constant, but voltage can change constantly.
Let's compare that to the local home with its 150 (or maybe) 200 amp service. The entire system is grounded at one point (often the water piping system, as noted in another cheerful thread on this site). Let's start with the 7 or 8 items plugged in at the receptacle for the HT system itself, hopefully not already kicking the breaker off at the rated 20 amps. We'll assume there is no power conditioning at the hair dryer, since a even GFCI receptacle is looking for current, not voltage. We will assume the bathroom is on a different circuit breaker than the living room. We will further assume the main breaker box probably has no line conditioning and thet various circuits are merely tree-branched at the main box, as that meets normal city "code". That leaves the hair dryer, kitchen stove, three ceiling fans, icemaker, and the air conditioner all isolated from the HT system by a couple of on-line breakers and about 50 feet of pure copper wire.
Do you have dedicated ground poles for your HT system, Mtry ? Any isolation transformers scattered around the house ? Maybe you're just running off your own personal generator (though that only eliminates problems on the main line, not inside the house).
If I'm having problems, you can bet you are too.
A voltmeter will be of only marginal help in this situation. A single snapshot is time is hardly an effective measurement. You would need a 24 hour, or even better, week-long voltage recorder to accurately represent the varying uses of electrical power within your house (suppose your clothes dryer, ice machine, and hair dryer were all on for even a short period of time), block, neighborhood, weather conditions (700,000 air conditioners all on at once in Dallas in August MIGHT cause just a little variation) and city.
If you are in an area with unpredictable electrical power compunded by varying internal use, and want a clean electrical feed, there is simply not a more cost-effective method than a battery-out UPS.