I recently noticed that my home was not being cooled when the temp outside was only 80. Had freon added as I have in the past (slow leak, cannot find it). Unit worked great that night and cooled as expected. The next day the unit ran, but would blow cool air for 20 minutes and then warm air after that. It would start blowing cool air again after cutting the unit off for 1/2 an hour or letting it continue to run (cool, warm, cool). Any ideas what may cause this? Could it be dirty coils?