I don't think engine power was affecting the altitude setting, the setting wasn't initialized so it was a ridiculous number like 2965703.15423549. That's why it went up. If you selected Alt-Hover first, the Alt-Hover routine initialized the variable and Stable-Hover would afterwards lock onto the Alt-Hover setting. That behavior was confusing the issue. I almost always go into Alt-Hover right after takeoff, so I wasn't seeing the problem later when I went into Stable-Hover mode.