A/C Blower won't come on when it's warm outside.

I have a 2001 Chrysler Voyager. The A/C & Heater work great...until the temp outside gets above 70 degrees. Then, like clockwork, the blower won't blow any air (however if you put your hand to the vents, they are cold or hot depending on cabin thermostat). I've actually watched the temperature go from 69 to 70 and immediately the blower will kick off (while the compressor remains fully engaged). Every once in a while, usually if I hit a pothole, speed bump, or slam the door really hard to jar the vehicle, the blower will start working again regardless of how warm it is outside. I've checked all the relays, which all work fine. This isn't much of an issue during the winter, but it becomes a big problem in the summer (I live in South Louisiana).