Back in the goold ole days (when my plane could fly) I found that I could stop the cruise porpoising if I turned off the electric trim.
I would say that perhaps it was a computer interface problem between the old STEC 60-2 and the new Garmin G-500....but I was porpoising before I upgraded.
I hold lateral course flawlessly with great cross wind compensation.
I have an ancient analog altitude pre-select....by that I mean that I have individual verticle mechanical dials that I use to input Baro, desired altitude, and desired verticle rate. Its primitive, but it works very well.
My only other nag is GS Intercept. In my bird, when I set it to intercept the glide slope I expect to hear a klaxon horn followed by an announcement over my intercom to "Dive! Dive! Dive!" cuz she immidiately pushes forward about 15 degrees nose down.
Fortunately, my thumb never strays too far from my Autopilot Disconnect button when I am below 3k
I have no doubt that my issues are some adjustment that needs tweaking.
But for now...I have other priorities.
Cole