I had the wild vents problem for a while, then the vents COMPLETELY stopped working. I could only turn on the defroster, or OFF. That is to say, no mattery which position I put the selector swtich in, it activated the defroster vents.
I just fixed it, after many hours of going through vacuum lines. Frustrating, all my check valves were operational, and the only leak I found was a disintegrated elbow-connector in the vacuum line under the battery (connects to the cruise control canister, I believe). Fixing this did not fix the problem though.
What was it? A LOOSE vacuum connector (multiple-line connector). The white connector behind the dash panel, that goes from the selector switch to the vacuum lines that pass through the firewall. (yes, those are vacuum lines, not electrical wires! It took me a while to realize that! ) I just unplugged the connector, and plugged it back in (took some real SQUEEZING to get it seated properly, all those gel-connectors inside of it that form a seal around the tiny little vacuum nipples inside the connector).
Now I can select defroster, dash, or floor vent. I don't know if the 'wild vents' problem is fixed though, until I drive it (that is, the problem of it changing vents while driving uphill).
I have a 2000 Dodge Ram 1500 Van. 5.2L engine V8.