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I'm not sure but what your experience is fairly normal. Often, on a long uphill climb with the accelerator floored, my vents will change just like yours. As soon as I let up on the accelerator, the vents change back.
The actuators for the vents require a constant vacuum. With the accelerator depressed, the throttle plate opens and the intake manifold vacuum drops way down. The check valve is supposed to prevent the higher pressure in the manifold from bleeding into the heater/AC vacuum reservoir. If there is even a tiny leak in all those hoses and actuators under the dash or in the check valve, the vacuum in the reservoir is "used up" and the actuators return to the rest position which is defrosters and floor. Since you've replaced the check valve, it seems that there is a small leak in those tubes. This would be expected as they age and harden.
In my case the problem is slight and I can live with it. If your system changes over too often, you may have to go under the dash and replace all the vacuum hoses and maybe an actuator or two. I haven't done this on a 900 so I don't know how hard a job this is. I have heard that some of the actuators are a PITA to get to.
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