I have had several of these "faulty alternator" problems causing the tacho to fluctuate. The "fault" is always the spade connector not making a good connection on the W terminal. Remove the plastic sleeve that covers the female part on the wire going to the loom and throw it away, clean the spade terminal, tighten up the female part of the connector with pliers so that it is tight to fit, and it will not give anymore problems.