panasonic microwave NN T551?

will switch off after 29 seconds all other things ok, clock, turntable,light, did have it running much longer cooking casserole, then next day it just kept swiching off no matter what time i put on.
It is still in tip top condition and I have taken great care of it. just want to know if this is a minor thing as don't want to have to buy another being a pensioner.Can anyone give any idea of what can be wrong I would like to know if anyone else has had the same problem if so was it able to be put right at not too big an expence .