a/c stops/starts intermittently

I started my 2008 (Gen 2) Prius 4 door a few days ago and the a/c would blow hot air (both sides of car) but no cold air. This is Missouri in June, 85+ degrees so I was concerned. Finished my chores and drove home. Next day I start the car and the a/c works just fine. Now I am confused, since I performed no maintenance... in fact, I did nothing at all to fix the problem, but now it's cool while driving around doing my chores, so I'm happy. Next day I start the car and again the a/c only blows hot air. So I check the a/c 10A fuse, and it's fine. The hot air is coming out pretty hard, so there is no problem with the blower motor or the blower filter. So I'm thinking about checking the "air flow gate" controller next and maybe finding out how to check the freon level after that. But today I start the car and the a/c is immediately blowing cold air again. I have no idea what is going on or what to check next... anyone else ever have this situation? If so, I'd really appreciate knowing how you fixed it. Thanks!