The content of the websites you visit also has an impact on speed; like I said, so many variables. It could also be your dish. The reps here can check your modem and see what it reports, and try to figure out if there's anything wrong with your system or whether it's just congestion (which the pattern in which the speed changes seems to indicate; congestion may be at least part of the problem).
What beam are you on? Is it 101? I'm on Bean 68 and I have issues with speeds being much better a certain times, and changing to very low speeds at other times. I experience patterns very similar to yours every day, and have now for a while. I'm hoping the new satellite will provide a solution to this issue.
Hopefully one of the reps will get back to you soon.