I woke up yesterday to find that my freezer was half thawed. The evaporator fan was not spinning.
I opened the evaporator compartment and found the top portion iced up, but the fan could spin freely.

After unplugging the fridge, and doing some troubleshooting (see below) I plugged the fridge back in and the fan started working.
The freezer got cold enough to freeze ice, and everything seemed to be working.

I woke up this morning to find the evap fan off again, the freezer half thawed, and the same iced up top coils.

Here's the stuff I checked previously.

Evap Fan: 0.5 Ohm, 120V when on
Defrost TStat: Closed (while on frosted pipe), measured <0.5 Ohm
Defrost Timer: motor spins, and turns compressor/evap fan on/off
Heater Element: 30 Ohm, 120V when on
Compressor Coils: (S-C)10 Ohm, (R-C)2.6Ohm, (S-R) 12.6Ohm
Start Relay: 4 Ohms
Overload: <1 Ohm
Condenser Fan blows when expected

I was about to overnight a defrost thermostat, but figured I'd ask around first.
It's obvious from the wiring that it has been replaced before.

I can only guess that the defrost t-stat has gone intermittent or the fan is intermittent.
I need to try and measure the voltages at the fan terminals the next time I find it stopped.

Any other thoughts?

Update

I replaced the defrost thermostat yesterday evening and closed everything up.
I found the evap fan this morning still running (unlike the previous 3 mornings), but the freezer wasn't very cold.

I spot checked the walls with infrared and measured 34-35degF.
The fridge was reading about 45degF.

If it still isn't frozen by the time I get home, is it likely a compressor pumping issue?
I was told that if I had a freon leak it wouldn't cold at all.