When using AC for the first time this year, worked great for about 15 minutes then stopped blowing cold air. Turned the switch to vent. Turned back to AC after about 15 minutes and it worked again. Again it stopped working after about 15 minutes. Sometimes it will work for anywhere between 10 and 20 minutes. When turning the vehicle off and restarting, it works then too. Again for anywhere between 10 and 20 minutes. Each time, I turn the switch to vent and then back after a certain period of time. What would cause the AC to shut off after a certain period of time?