First of all Ron, you might consider changing the title of your thread to something that suggests the possibility that your Trio unit is not set up properly.
In my experience (RV-6 which is known for being very light in pitch, especially loaded) the EZ-Hold will hold altitude very tightly regardless of how the plane is trimmed (unless it is
waaaay out of trim, then the "Clutch Slip" message will appear). The fact that you are having to mess around with trim and that the unit makes abrupt corrections leads me to believe there is something not quite right in the installation. Does your elevator control have
any stiction in the system? From your message it sounds like the autopilot is having to overcome friction or hysteresis in the elevator controls. You mentioned trying different combinations of gain settings, but your description leads to conflicting conclusions on that point. Higher gain will result in more abrupt corrections, but would not allow the plane to deviate 20-30 feet before the corrections occur. Low gain will result in soft corrections but may allow the plane to deviate a little in less than smooth air.
But you are describing altitude deviations with abrupt corrections. This is counter to customary gain adjustments and EZ-Hold behavior and leads me to believe the system is working against inconsistent forces in the elevator controls. Just a tiny amount of hysteresis near neutral would cause the servo to hunt. Your efforts to secure the servo were mentioned so I assume the servo isn't moving in its mount and all the attachments are tight.
Here is something you might try. Disconnect the elevators from the pushrod at the back of the fuse. Make
sure the elevators have absolutely no friction around the neutral position. If the elevator hinges aren't aligned properly there can be some stiffness as the elevator pass through neutral. This probably wouldn't be very apparent while handflying the plane, but to a servo that is trying to make extremely minute adjustments, it could be significant.
One more significant item to check; Is your EZ-Hold plumbed to the plane's static system? It needs to be connected to outside static and even the tiniest leak can skew the system.
Your experience isn't consistent with many other EZ-Hold users I am familiar with and that is why I am suggesting taking another look at your installation. Hopefully this will lead to something productive because the EZ-Hold is a great product and you should be able to enjoy outstanding performance with it.
(I don't work for Trio, but I have quite a bit of experience with their systems.)