My Carrier 38CKC central a/c unit stopped cooling during the recent high-90's weather. The fan on the external unit runs when the thermostat is set to turn the a/c on, and the compressor sounds like it tries to start for a second, but then it immediately stops. It tries again to start about every 30 seconds, but never succeeds. The unit is about 7 years old. Any thoughts on what's going on?