Is it the groove in the bottom of it causing the problem. The thing that rotates and the turntable sits on?
I bet they changed vendors for that or that rotating assembly had changed. Microwaves cause nightmares for me. When our microwave/range hood thing went it about four years ago, we ended up with an whole new kitchen and out about $26K. I tell people all the time - when you buy a built-in microwave, buy three extras and hide them. When one goes out, sneak in the middle of the night and change it out with one of the spare units. They will be money ahead.
Way to stay after them!