Driver's side heat changes to cold

After about fifteen minutes of driving, all outlets on the left side switch to unheated outside air regardless of temperature setting. It will switch back to warm air for a few minutes after restarting the engine.
For both sides, a setting of 78 gets to about 70. The A/C works okay.
The outside temperature display usually reads much warmer than it is and sometimes rises high then plummets when A/C on. Outside sensor replaced but no change.
Do I need a new main control or just a left blend actuator (and how is that accessed)?