I suffered this a lot until recently with various aircraft. Any kind of weather mode - other than windless "fair weather" would keep kicking the plane out of the altitude hold.  Problem was is was always downward such that it would bump up against a kind of invisible ceiling and essentially step descend all the way to the ground. Most annoying.

The solution was my Saitek X-52, which "auto calibrates" each time you reboot your PC, was incorrectly calibrating the stick each time the PC rebooted e.g. for MS updates etc because I had left the knob/slider I use to control pitch trim out of the neutral detent.  I make sure now before a reboot the pitch wheel is in the right place and no more alititude hold problems.