Sorry to hear about your ribbons failing. Frustrating.
I've had my 20.7s for over three years and they are working flawlessly. I even changed the tweeter fuse for one with a slight higher current rating than the original(3.0 amps instead of 2.5 amps) and I have not had any failures yet.
You might have a flawed crossover that sends too much current to the ribbons.