Cat D6K2 AC question

The air conditioning in the D6K2 I run (Serial # CAT00D6KAWMR00579) has been problematic this summer. On cooler days/mornings (70° F +/- 5°) the evaporator freezes about every 1-2 hours, restricting airflow. On warmer days it will barely cool, sometimes better, sometimes worse.
Here's what I have done so far.
- Replaced cabin air filters. These are regularly cleaned and replaced.
- Exposed the evaporator coil and made sure it was clean and the temp (?) sensor was embedded in the coil....

Cat D6K2 AC question