I've just about decided that they periodically slow down the Internet speed in order to get you to purchase the higher speed.

I don't even know if that's possible....but it sure seems like something they'd do.
I've gotten to where I call in each and every time it gets slow or goes completely out because I want it documented that I've got sub-par service. Because I will ask for a reduction on my bill.
Sally, if you threaten to change providers they'll send you to their Retention Department where they'll offer you all kinds of stuff to keep you with them. It might get you a better rate......which is not entirely unjustified if you're receiving bad service.