Download speed issues from ISP

My ISP is TWC. Been having speed issues on and off for about 6 months now. New residence. Some days my down speed is just fine and i get what I'm paying for (50mbps down) and other days It'll just take a dump and bottom out anywhere between 3-5 mbps down. I've contacted TWC support multiple times, even had techs come out to the residence mutiple times and whatever they do it's always a temporary fix. I've contacted them at least 10+ times about this issue and it still has never been fixed. They've replaced my modem, tested my cable lines. OS is Windows 7. Modem/router is a Ubee DDW365, it is only a week old. Having the issue from ethernet connection and wireless. I've tried multiple ethernet cables, no change. Multiple wireless devices, no change. The issue occurs even when i only have 1 device connected to the modem/router. I've ran ping tests, packet tests...no issues there. The problem with TWC is they will just stop diagnosing the problem when they see I've got satisfactory download speeds. But almost always withing a couple days my down speeds have taken a dump again. I'm just tired of them providing me with temporary fixes and was hoping someone here might be able to help me with a permanent solution. Of course if could be something I have no control over and may never have this issue resolved until I can get away form TWC, but for now I am stuck with them unfortunately. Any help at all is much appreciated.