Using a shore power isolation transformer at home?

There are other methods of regulation but "servo" just means some kind of feedback and not sure what that means here. A household thermostat can be called a servo.

And not all tap switching implies a stepped sine wave, if it's done at half cycle.

Sorry, I forgot to tell what Monsters AVS 2000. Here is a photo: http://www.bukutronics.com/?p=54. Besides those "high voltage suppressor" and "radio frequency suppressor" (see here) which I will surely bypass before I use a unit, this is nothing but a variable transformer (variac) with a motor that rotates its pickup contact, trying to keep sharp 120V.

The Monster bastards charge for a new unit 4-5 times more than what it is worth; besides, they almost surely limit the current by their "suppressors". However, to buy such a unit for under $500 (btw, the variac is 3 kW rated) and to slightly mod it into something less limiting starts making sense, at least to me. Of course, this type of a device does not give an instantaneous voltage correction if something serious happens to the AC line (that is why I want a non-limiting surge suppressor from PS Audio after the variac), but what I have most of the time in my place is a "slow floating" somewhere between 118..225, which is driving me nuts! To give you an example: those fluctuations are wide enough to cause the filaments on my 45's to go all the way from the allowed minimum of 2.45 to the allowed maximum of 2.55. Wouldn't you find that annoying?

What you are saying about transformer vibration is right, let me tell you what I have narrowed it down to. Two important things: first, it happens relatively rarely, and usually at nights. Second, similar thing start happening to other electrical devices - at least, to the ones connected to the same AC line from the panel. What I have not done, unfortunately, was to check whether other AC lines from our panel suffer from this also - probably, will do it as soon as this humming happens again (btw, the hum is rare but very obvious, nothing to blind-test here). To conclude, what I know for sure is that the transformer vibration is caused by something that occasionally comes from the AC; what I do not know yet is whether the cause is inside our house or something outside.

Seriously, mechanical? How Victorian. A modern classic. Hope you like cleaning and replacing brushes. Do you have to grease it or just refill the whale oil?

Oil change every 10,000 kWh, as usual :)

I don't understand how an isolation transformer would help with voltage fluctuations.