Ford fusion-Blower and A/C keep turning on and off

The blower and A/C on my 2006 Ford Fusion keeps turning on and off. The system has manual knobs and is not computerized. It turns on just for a few seconds and then turns off. The knob that controls the turning on and off of the system is in the off position when this happens. What could be causing this? Thanks for the help in advance.

Does the knob have various speed settings? If so, does it not work in all positions or does it only consistently work on high?