If you had said you had just fitted a new one I would have guessed that the pole switch was in the wrong place. I;ve not heard of the switch randomly changing, but you may have a fluky vibration that has moved it.
The tacho counts alternator pulses. if you have more poles in your alternator than the tacho thinks, it will for example see 4 pulses per rev instead of 2. If your switch is set to "4" it will be expecting 4 pulses per rev. If you have a 6 pole alternator it will in reality see 6 pulses per rev. so the tacho will think the engine is going 50% faster than it really is. The only way the tacho knows how many pulses it should see is by that switch on the back.
Did it just randomly start doing this, or have you been mucking about?