Side by side Frigidaire 6 months old. Has done this before, but was working fine by the time the repairman showed up. It is NOT overloaded. It does usually run a degree or two different, but not 9 degrees.
It sounds like the evaporator coils are freezing up and the fan is unable to keep the air moving, it could be a faulty defrost timer, this is unacceptable for a new fridge.