is this a problem altenator gauge moves around??

we have a GQ patrol, with the 4.2 lpg/gas auto, our first altentor was putting out to much current and burntout parts of the electrical system with a power serge, then our second altenator was fine until all of a sudden it was reading too low, so now we are on our third altenator our problem is the altenator gauge moves around, sometimes it stays put, but when revs are low (normal idle speed) it drops down to 12 volts, but if the revs are higher it picks up to a normal reading, and sometimes while driving along the gauge will read normal and all of a sudden will read up to nearly 16volts for a while then back to normal again these reading have been checked with the multi metre (where we can) and have been right. my question is could something else be causing our problems as pretty soon there will be no altenators left as we would have tried the all. any reponse or ideas of what to look for welcome.
chris.