I like the suggestion of a sticky/failing switchover valve. With the glovebox removed you could try tapping it while it refuses to activate the recirc flap; that might help isolate the problem. With the glovebox removed a very distinct, single click noise can be heard as the switchover valve activates. Should be able to hear this when activating the recirc dash switch.

The only other idea is a crack in a vacuum hose or rubber elbow. Sometimes it'll hold enough vacuum to pull the element closed, sometimes not. I've seen similar operation from old vacuum elements with the diaphragm on the way out, but that can't be happening with brand new elements.

Mostly I'm clueless. I hate intermittents. My approach to solving intermittent problems is to wait until they become continuous problems...

- JimY