My 300SD has an intermittent (usually off)flow of air from the center vent. This occurs when the pushbutton is on Auto Hi or Lo.
Flow is fine to the side vents. A/C does need a charge of freon. Compressor switch seems to have no effect on this.
Air flow appears normal with pushbutton in Bi-level position.
Unit worked normally most of last summer, then air to center vent just stopped. Air flow OK for winter heating. Tried A/C this summer and, again no flow to center vents. Last week, got flow to center vents for a day or two, then did not work again.
Would appreciate any advice or suggestions on where to look for problem, how to test, etc.
Thanks for your help.

Steve