Odd multiway light switch problem

In our 70s vintage house there is a light switch on the wall (not near the door) with a piece of red tape on it, obviously saying "Do not switch me". I decided to see why. It turns out to be one of two switches (the other at the door) controlling the main room light. I tried switching it when the door switch was off. The light came on briefly as the switch passed the midway position, then went off again when the switch was fully switched. In other words, both positions of the switch left the light off. Hmm. I removed the screws holding the switch plate, and replaced the switch with an identical one. Same behaviour. Hmm. I then discovered that I could make the light go on and off by moving the switch plate slightly (when off the wall). Sounds like a wiring fault, but unfortunately the wires between the end of the sheathing and the switch are clearly intact. Hmm. Out with the voltmeter, to test the voltage between phase and earth and see if it is affected by movement of the cable. This is where it gets really odd. The voltage between the hot wire and earth cycles between 230v and 190v with a period of about 7 seconds. WTF??
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On 12/05/2013 12:26 p.m., Gib Bogle wrote:

Correction: looking at the circuit for a Traveler or Common system (which I presume this is) http://en.wikipedia.org/wiki/Multiway_switching I see that of course there is no earth wire. I don't know what the wire colours signify, but since there are only three conductors at least two of them have the potential to be hot.
I need to think about this :-]
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Gib Bogle wrote:

You are probably measuring with the lamp in series.
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On 12/05/2013 1:22 p.m., F Murtz wrote:

The lamp (fluorescent) is in the circuit - I'm not sure why that causes a fluctuating voltage, but I'll believe it.
Anyway I now understand the odd switch behaviour. It's just the crappy 70s switch design. The replacement one I tested with is the same vintage, no doubt installed somewhere in the house at the same time. The contact made by the spring action was fine, but the contact made by the rocker switch depressing the contact arm was not good. I have cleaned the contacts and bent the arm slightly - now it works as it should.
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On 12/05/2013 1:22 p.m., F Murtz wrote:

Another correction. I now have the switch working OK, but the circuit wiring is odd. One of the two incoming conductors is always live, unaffected by the door switch position. Now I have taken the door switch plate off, and it has far more conductors wired into it than can be explained by the two room lights. To my great surprise I have discovered that the circuits for the front door light (outside) and the hall lights are all powered from connections made at this switch (of course they have their own switches). The way the thing is wired up it is not functioning correctly in multiway mode for the bedroom light. Very odd. I will try to sort it out.
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It was definitely mis-wired, so the multiway system didn't work even when the switch was fixed. I finally figured out how it all should go together (not at all complicated, in reality, but sort of new to me), and now it all works.
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