[billybek]

Check the easy stuff first.
Door seals are not surprisingly a big deal.
Take a dollar bill, (or a 5 dollar bill in Canada) and close the door on it and try to pull it out. If the door seals are making good contact, there will be some resistance.
Put your hands under the doors and feel for cold air falling out.
Air that falls out needs to be replaced with warm air that has moisture in. Now you are sensibly cooling the new air and drying it out which puts a high latent load on the system. Frost forms on the coil that acts as an insulator and reduces heat transfer. All those things increase run time.
Easy stuff first. Door seals, condenser cleanliness, condenser fan operation, evap fan operation, frost on evap, evap fan.
Is the frost on the coil being melted away in a defrost cycle? No?
Defrost heaters or defrost timer or controller. Possibly defrost termination switch.
Compressor? Cold suction line, warmish liquid line, hot discharge line.
The very last thing I would do is line piercing access ports if the compressor doesn't have them.
They are leak potentials and you lose a decent amount of refrigerant every time you hook up a gauge to them.