Since you found it speeds back up on resets, are you doing that now?
At my home (not to mention other service calls) I found that out of date router firmware can be a factor. Also if anyone is running what is known as Torrents then you find no support most of the time. It's something that is well known to kill speeds.
I've had problems with Cox internet service (southern california, not a rural area) for past 6-8 months, hardwired and wifi. Service starts to slow at about the 3 week mark, I call and complain, they either reset my modem or send someone out and miraculously it works great for another 2-3 weeks. I've been told my house is too large (3000 sq ft) and has steel beams so I purchased a 2nd router so now there's one upstairs as well as downstairs but even my computer that is hardwired slows to a snails pace. We are limited on ISP providers in our area:( Cox has assured me that they don't slow down high data users as that is illegal but I'm honestly starting to wonder. Can anyone give me any advice on how to fix this? I've had the Cox people out every month for past 6-8 months. Thanks for any advice!