Why is my Samsung SR-S2026CVW rising 10 degrees?

I have a Samsung SR-S2026CVW and has recently had a new defrost heater fitted to fix defrost/ice build up problem. This seems to have sorted the problem out but now the freezer reaches target temperature of -20 and then compressor switches off and the freezer temp goes up to -10 and then compressor kicks in and goes back down to target temp of -20 and repeats this cycle all the time. Freezer temp is always way below -0 but some things do thaw a bit and then freeze again which is dangerous. What can cause this? Is it the thermo fuse or defrost sensor or something else entirely. There has been £200 spent on this fridge freezer on repairs but it still is no operating properly. Has anyone had this problem.