My a/c keeps switching to blow only out of the defrost vents.

What is your question?

Checked for vacuum leaks, can't find any,recently put a stereo in.My boyfriend is pretty good with cars,he also checked the switching doors and they seem to operate. It switches back and forth for no apparent reason, I even tried leaving the vent control on the same setting after it was coming out of the front vents,but it randomly switched back to the defrost vent again. Its too hot for this :-[