POSSIBLE SOLUTION: When I replaced most of the vacuum hoses in the engine bay, I didn't make the combi-valve hose long enough. Yesterday while fixing something else I noticed that this hose was 1/2" too short and crimped.

Switched it out with a longer hose. Now I just need a cold day so I can test out this possible fix. Hope I don't have to replace those combi valves because they look like a b*tch to get at.