I have a very intermittent problem. If I have the AC on for more than 30 minutes or so, cold air stops blowing in the front vents. Back vents blow fine though. Last time this happened, I turned the system off for 10-15 minutes and turned it back on. AC worked again although not as well, but well enough. Also, if I have the AC on pretty strong, I don't have this problem. Any ideas?