The nature of gravity coils....no air movement in the box....can easily result in what you are describing. The moment someone enters the box, the temperature will increase. With no air movement it takes a long time for the heat transfer process between the air and refrigerant, and depending on gravity to circulate the air and get even air temperature takes a long time too.
Unless the thermostat is mounted substantially above the product level it's hard to understand why the product is freezing...unless the stat isn't accurate.
It would be interesting to check the temperature at different heights....say, after a defrost period....to see how much variation there is between where the product is located and where the stat is located.
What is the product they are refrigerating and how often are they in/out of the box?
Good idea to add the head pressure control valve in a climate like Colorado. I'm with the suggestion to replace the gravity coil with a tradiional fan/coil unit. There's a reason why you rarely see a gravity coil in a walk-in box.