Hmm, I quite possibly did not phrase my question too well, in which case I apologize :P.But since this problem is still plaguing me, I guess I'll give it another try.
So I'm flying my plane manually right now at my assigned altitude (5000 feet). I punch in an altitude of 5000 feet into my autopilot, and activate the AP and the altitude hold functions. But my plane will not stay at altitude, instead relying only on my elevator trim inputs. It's just like flying manually, attempting to trim up and down just perfectly (next to impossible). It's not just regular oscillations that take place under AP's flight envelope, it's raw trim and will continue either up or down just as if under manual flight.
I guess I just don't understand the pitch well enough. Should I centre my trim before going into autopilot? If I do that, I lose the trim that was keeping me level. Arrrghh. It's probably something stupid (very stupid) I'm missing but in weeks of flying I haven't figured it out yet.
If anybody has any input at all, it would be much appreciated.
What's your airspeed? If you are flying in level flight that's approaching "clean stall speed", then your AP won't be able to do much. At 5000 feet, you may have to have either less than maximum weight, and also have the air/fuel mixture changed from sea level, to get the airspeed required.
Now, you shouldn't need to center the trim before turning on the AP, although it doesn't hurt, and probably would in fact help make the transition a little smoother... but the AP "flies" the plane using the trim functions anyway, so if your plane is already trimmed when you turn on the AP, the change should be minimal.
That said, I wonder if the problem/solution is much simpler than that: make SURE you not only have the AP actually activated(left-most button "AP"), but then actually activate the "Altitude Hold" on the AP too (right-most bottom button "ALT"), as it's not on by default.