I am not an electrical boffin but the following springs to mind. Are you only monitoring the voltage on your Garmin's? If so the both problems, the RPM reading fluctuations & the high voltage problem may be solved as described by jetjr.
However, if you have a faulty regulator you could very possibly fry some serious equipment (big $$$)
I would suggest you connect a multi-meter or if you don't have access to one a simple VDO volt meter to your electrical system and check the voltage you see. If you are seeing 18+ volts on your Garmin's and the multi-meter agrees, you have a "real" regulator problem and it needs to be sorted immediately. If on the other hand the Garmin's read 18+ volts, but the multi-meter reads +-14 volts then I would guess the Garmin's readings are wrong and the problem may be sorted as per jetjr's suggestion.
How did you get one with the TOCA, very interested in your feedback on that.