This is EXACTLY what happened to us!! And, we had direcway for YEARS without any real complaints. I have seen this all over the internet.
Not only did we upgrade the modem, we upgraded to a bigger plan too, so we could have even more speed and limits. HA! We have less now than we ever did, and they will not allow you to re-commission the old modem!
I think the FCC should look into them saying they can provide these services when they obvious cannot handle the clients they have.
They seem to cycle the service down to areas at different times for a short while, and then POOF your equipment is mysteriously fixed again (since they keep saying it has to be the equipment). They are not suppose to restrict you, if you do not go over the FAP!
It's ridiculous. But, they get away with it, because those who do have their service - generally cannot get anything else. So, it's take it or leave it. But, it shouldn't be legal.
I hope you do have the means to sue them. I wish you ALL the luck you can imagine!!