Blower on high on defrost is the default mode when the HVAC control unit is not functioning. Based on it turning on and off there is probably a loose electrical connection between the body harness and the control unit, or a loose component (failed solder joint) inside the control unit.
Pull the HVAC control until forward without disconnecting it, and wriggle the electrical connectors at the back to see if you can get it to turn off and on. If that doesn't work, you might want to take the control unit apart to see if you can see anything obvious. If you aren't able to do that, or don't see anything wrong, you may have to replace the control unit.
In my experience, failures inside the control head are not intermittent; once they fail, they fail. I would start by focusing on the electrical connectors at the rear of the control unit.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)