Fisher & Paykel N210L freezer doing my head in

My F & P upright freezer is driving me nuts.
A few months ago it wouldn’t get cold enough. I traced it to the defrost heater element. I replaced the element and it was good for about 6 weeks. Then the kids started complaining about the ice cream being too soft. I pulled the cover panel off the inside of the machine and the condenser was totally iced up. I checked the heater element. It’s OK. I removed the defrost timer and checked it with a multimeter. It checked out OK. I melted the ice and put it back together. It worked fine and the inside temp was -18C. A week later and everything is defrosting again. The inside temp is now +5C! This time there is no ice on the condenser.
I set the thermostat (Danfos 077B2225L) to 7 (max cold) but the compressor doesn’t run (I rotated the timer with a screwdriver to make sure it wasn’t in a defrost cycle). If I bypass the thermostat the compressor runs. Am I looking at a faulty thermostat?
Appreciate any advice.

sorry, I meant evaporator not condenser.

Sounds like a faulty stat, try a new one on it.

OK thanks, I'll do that.