Maleficio, it occurs to me that your car should register around 14 volts at all times that the engine is running. A small drop is normal when you turn on the lights, but if you are having this much voltage change over the RPM band, then something is wrong.
-The voltage gauge on the instrument cluster isn't that accurate. It is good for trend information only. That is, if it suddenly is really low one day, then you know something is wrong. Your best bet is to access the battery, and using a voltmeter, test the system voltage at the contacts engine off and engine on. It should register around 12 volts engine off, and about 13.7 volts engine on, even at idle. If this is the case, then nothing is wrong but your voltage gauge is probably shot.
If it shows what you have mentioned, then either your alternator is failing or its' voltage regulator is failing, or some combo of the two. Clean all the contacts. Walley Pendley has a good write-up floating around the internet, and there is more info on the Nichols page.
http://www.nichols.nu/tips.htm
Good luck!
N