I had something similar with my Bosch micro. Take the plate out and turn the oven on at low power for a few seconds. The point is to see if the plastic piece in the centre, which the plate locates on to, is actually turning.
If it is then it may be that the plastic piece is slipping on the motor shaft when the plate and food are in the oven.
On mine I was able to pull out the central plastic piece, give it a few drops of super glue, and put it back. It has worked now for many months.
A word of warning - this is a do or die solution. Once superglued on the plastic piece and hence the motor would be very difficult to replace.