W124 Center vents no A/C Climate Control ?

[HalBodin]

I have a 1992 300D 2.5 Turbo. It is a great car and a few days ago the center vents seem to not work. However, in the morning (cooler weather) they seem to work. At least for a while. The center vents work fine on "vent"... defrost works too! Any ideas? I hope that it is something that may be a minor repair and not a major repair...