When I have overcooked (Varycon, Polywarmtone, Adox VarioClassic, Ilford MGIV) I have done nothing more than burn a hole in the paper. Perhaps it is paper brand specific, or perhaps it is your particular microwave.
I bet it's the microwave oven. They vary quite a lot in the evenness of their microwave distribution. With some, if you put in an even layer of something (say, pre-cut cheese slices), you'll find some are stone cold while others are bubbling away. With other microwaves, the distribution is much more even.
Using a rotating platform sometimes helps. If you've got an oven without one, I know that they exist (or did exist) as add-on things. I've got one in mine that you rotate and then it slowly rotates back, like a wind-up toy.