1994 c220 a/c problem

[starman629]

I have a 94 c220 the air from the center vent goes away the moment I put the car into drive or reverse and if I put it back and hold alittle gas is starts to blow, also if I am in park but turn on the recirc button the center air goes away. While driving under acceleration it seems as if the fan slows down and not much air is coming out. Checked for leaks and had the vacume control block replaced but still no good. Any ideas?