I have an issue that crops up maybe roughly half the time, where I set a target altitude in the autopilot, climb out in NAV mode with altitude armed and VS mode, and find the vertical speed less than I set, and gets lower as I go higher as if I was in some IAS mode. When it gets to the target altitude, it sort of levels off but keeps climbing at maybe 100-200fpm.
In this example I set the target altitude to 8500', and it has way blown past that and keeps climbing, even though in this case I set VS mode to -600fpm. I thought maybe trim was maxed out for some reason, but at this time the trim was less then 1 back from neutral.
On other legs, AP works exactly as I expect, no problem.