I second Elizabeth's suggestion about the windows having at least a partial effect (without knowing anything else about your room). I find that opening the windows has a noticeable impact in my room--it's a less than ideal layout with lots of glass, so my effect may be more pronounced. At any rate, I believe having the windows open basically "vents" the higher frequency reflections that contribute to smearing and grit.
But I'm also pretty convinced that power quality (both time of day, and proper, synergistic conditioning) will contribute as well.
But I'm also pretty convinced that power quality (both time of day, and proper, synergistic conditioning) will contribute as well.