To maintain a set fan speed, just manually select the speed you want and it will not change, but the temp will change to maintain the set temp. Remember you can also manually select inside/outside air. You can not control the vent temp, only the cabin temp. You could play with the temp setting and fan speed to regulate vent temp and the vent temp would still vary a little. If you think about it; if you have a constant vent temp the system (like the old systems), you would have to adjust the temp as the interior heats up until the vent temp maintains a comfortable cabin temp. This is what the system does automatically.
This issue comes up with new owner until they have had time to use the system more. Again, the system works best when you find a comfy temp and set it on auto. It really does work great.
Koz