Returned item giving same problem after fixed

I bought a microwave last year and 11 months down the line it started giving problems like it is pressing itself .no botton is working .i returned the microwave on the 16th of november and it was fixed i collected it .and now ater 2weeks the microwave is repeating the same problem again .i want back with the microwave and they told me they will book it againd and i am going to get the microwave 9jan. Now the problem is that i trusted them that they were going to fix my microwave and now they didi deliver so what next they just gave me a temporary solotion and i ca not even trust the microwave may again when they fix it it is going to give me the same problem .i do'nt know now am vert dissappointed because i can not buy a microwave every year.