I have a Frigidaire top freezer refrigerator. The freezer is fine, however it freezes up and blocks the vent, preventing cool air from circulating into the refrigerator section: Pic attached is freezer interior:

Problems:

• Ice builds up on back panel of the freezer.
• Air vent into fridge gets blocked by this ice.
• Fridge doesn't cool down.

A few things I have tried:

• Defrost heater is reading 35 ohms.
• I have noticed the fridge enter its defrost cycle throughout the day.
• Manually turning the mechanical defrost timer does send the unit into defrost.
• When I trigger the defrost, the heater does warm up and glows red.
• Ventilation fan is working.

What could be the problem here, or what should I check for?

The thermostat should be okay, since it turns on the heater once the defrost cycle is on. There is no problem with the defrost heater. The only other thing that could be the problem is the defrost timer. However it does activate the defrost cycle throughout the day that I've noticed. Could it still be faulty?