2005 Savana 3500 360 V8 - sticky(?) heat/cool control setting

[mad max]

Having problems with heating and a/c (blue-to-red heat/cool control dial) vent operation.

Previously this was found to be a block with a bunch of hoses under the passenger dash that had a small clear vacuum hose disconnected. Reconnecting it made the heat/cool control work again. But within the last year or two I've noticed that come winter of fall and I change the setting from heat to cool, or cool to heat, the vent(?) seems to stick for a while and then while driving on the highway in blaring heat or freezing cold, poof:the thing finally decides to switch over to the right condition. Winter is around the corner and it's stuck in cool again, which also obviously makes trying to control/fine tune the temp with the blue-to-red heat/cool control dial useless.

What would you think might be causing this problem: where should I look?

Thanks,

Mad Max