First time I have flown in my rv6a and I noticed the voltage meter fluctuating about twice per second between about 13.5 and 14.5. What is causing this?
Could be what is called a galloping alternator. Caused by resistance in the alternator/voltage regulator circuitry causing fluctuating voltage. It happens more often in older aircraft as oxidation creates resistance in various connections in the system. My RV-6 developed it about 5 years ago and a new alternator switch plus cleaning all of the connections in the charging system corrected it.
But, before I went too far down the rabbit hole, I'd get a cheap digital voltage gauge off of amazon and plug it into your 12V receptacle, assuming you have one of those. I'd use it to verify your readings.
Do you have a plane power alt? They seem to have a lot of problems with the connector. Following Kyles advice, this is where I would start if you have one.