Hmm, probably the story you got from Comcast is half true. Alot of times they call it it to maintenance, but unless you have a maintenance ticket to track it, its not a official thing. Its like calling up your town and reporting a pot hole. Eventually it will get fixed, but it doesn't mean a guy is on your street the next day paving. Then again, usually you get a pretty good answer from DSL reports, so if he gave it a green light, that is good enough for me...for now.
As for your graphs, could you give me some context? From what I can find online, usually NetMeter measures from your NIC, your WAN(Router) and your Modem. Which is that for? Also I can find documentation on how it graphs upload vs download. And last, whats the X axis. Hours? MS? Is that per throttling cycle?
Also you might want to look into a program called Pingplotter. Netmeter looks to give your a capacity test, which is good, but there can be many reasons for speed decrease. Now if your doing a ping test along side of it to a IP inside your ISP's network (say their DNS or DHCP server) and see a latency spike next to a speed decrease, then you have black and white proof where the problem is. Ofcourse this is only relevant when you have a direct connection between the PC running the test and the modem.
Let me know about those graphs. Visually speaking, it looks like there could be something there. That looks too steady to be environmental (wind, storm,temp,etc). And where did you get that info on "maintenance mode"? Sounds like theres more to that story.