Neff microwave combination oven

Question

has been working perfectly for the last two and a half years with no problems. about two months ago whilst using the microwave the main fuse inside the appliance blew. called neff engineer and he replaced the pcb and megatron, that lasted two days, he came and replaced them again and they lasted 5 weeks, and he has just been again and replaced just pcb and that lasted just 24 hours. do you have any idea that could be causing this fault?
thanks