Hello Nick, my name is Rick and I will assist you today
There is something out or order in the defrost cycle of this unit, it can be the heater itself, a defrost thermostat or even the control board cycling the defrost.
The reason you are noticing more often with higher heat and most likely humidity, is when you open the freezer there is more warmer air entering the unit, then during normal weather. If it is doing it now though it will continue to do this when the weather cools, just taking longer
When you defrost do you remove the rear panel to expose the coil?
When I defrost I do not remove the rear panel, just leave the doors open for a couple of days.
Would the malfunctioning defrost cycle cause the motor to be going most of the time. It is now not so hot but the motor still runs most of the time
It can play a factor, because when the freezer freezes up it is not allowing good air flow through the unit and the thermostats tell the compressor to run longer to try and compensate for the higher temps.
How old is the unit?
About 6 years old
The next time you defrost the unit, take off the back panel in the freezer. there you will see the coil, inspect the heater and make sure all connections are good. That is the easiest and the first step, unless you call out service