While I don't know your particular brand/model, the Apollo Convection microwave that Heartland installed in their products around 2010 had this same problem frequently, due to a thermal protect switch cutting out at too low a temperature. Apollo was exchanging a lot of units for that. My dealer got mine exchanged and installed the new one under Heartland warranty.
If it is convenient, 1) see if your dealer will exchange it for you under warranty 2) Try calling Heartland Service to see if they can get you an exchange unit to install, or 3) Call the microwave manufacturer directly about repair or exchange. My suggestions were made in order of forseen least cost/lpersonal labor for you.