how can i fix my neff microwave combination oven H5972NOGB/01?

my neff microwave combination oven has been working perfectly well for the last two and a half years no problems. about 8 weeks ago whilst using tne microwave it blew the fuse inside the appliance. engineer came and replaced the pcb and megatron, these lasted two days, he came again and replaced both again, these lasted 5 weeks, he has just been again but only replaced the pcb, this lasted only 24 hours!!! IT only happens when the microwave is being used? Can anybody please help? Thanks