I have a sharp micro/convection. It is shutting down after 59 sec, doesn't matter if I'm in micro mode or convection mode. Help.

Throw the sucker out. Microwaves are nothing but home radiation units designed to ruin your food. I threw mine out 4 years ago and have had no problems with my health since, Also, you don't want to repair any microwave unit as you can be zapped into next year from the built up voltage.