This might be "couple" of years later, but I recently just became aware of same "problem" in f-15C, after cruising around hours with f-22 altitude hold on. In F-15C (and many other planes) you also need / can trim at the autopilot modes. (I dunno if this is F-15C IRL stuff, maby) Or atleast this helped me with this very same problem (altitude drops faster and faster until hold releases itself..). I recommend trimming before and in autopilot mode. What really annoys me many times is that F-15C does not have trim reset. Thats one ED could add, more or less optional in every aircraft. That would help a lot while learning trimming in "tuff" places. Oh how many times did I drop my SU-25t after using autopilot
