Intermittent A/C

My compressor crapped out a year ago and was replaced with a refurbished compressor. A/C worked great for about 5 months until it started blowing warm air again. This occurred during colder months so I didn't have it looked it until May. Turned out there was a leak and all the Freon escaped. My dad put in more Freon and included a dye so that the leak could be found, and has since been fixed. However, now the A/C is working intermittently, blows cold air most of the time, but will randomly blow warm air for a few seconds or minutes. I've noticed that it seems to occur when I am sitting idle or decreasing in speed significantly, but will quickly begin blowing cold air once I speed up. Any ideas what would be causing this? My dad is a pretty good mechanic and does all my repair work, maintenance and this has him scratching his head. We are thinking electrical but not sure why it would only occur at low speed or idle. Any help is welcome!