Hotpoint Mistral Plus - freezer temp intermittantly goes above freezing!

I'm hoping that someone could advise me on a strange problem I'm having with my fridge-freezer. It's a Hotpoint Mistral Plus (not sure the exact model).

Mostly it seems to work just fine with no obvious problems. Freezer temp displays as about -20, and fridge stays close to its set-point of 4 deg. The compressor runs intermittantly only.

However, I've noticed sometimes that food in the top of the freezer has started to thaw!

I put in a max/min thermometer and left it for a couple of days. The temp at the top shelf of the freezer is rising to +6 deg sometimes, then returning back down to -20. I think this happens every couple of days, even if the freezer is kept shut. This is clearly not supposed to happen!

No error codes are displayed.

My theory is that this is caused by it getting stuck in a defrost cycle for too long, effectively turning the evap into a convection heater. This would suggest a problem with the evap thermistor perhaps?

Anyone else seen this type of problem before?

Thanks.