accasional climate control glitch

it's not a big problem (hopefully) and it doesn't always happen, but once in a while (every few weeks maybe?) we don't get refrigerated air.  it blows hot, cold whatever, but it is just air.  stopping then starting the engine then returns the cooled air.  I've asked on and off during the warranty but the dealer could never find anything wrong, although I'm not sure they got what the problem was, because naturally it always worked when I took it in,  thanks if you have any thoughts