Comcast XFINITY → [Speed] Horrible Speed ... intermittantly

For the past week, I have been getting intermittent horrible speeds. For example 140 KB/S download, or even worse 24k KB/s. It will be bad for an entire evening and then it will be "normal" 20+ MB/S for the next day. Just today the speeds went to sh*t again for about an hour and now are reasonable again. (Cable is fine during these problems) I have no idea how to proceed to figure out what is happening and how comcast can fix it. Any ideas or suggestions?

Just a followup ... I had comcast come and check my wiring. They tested the connection and everything seemed fine. He then changed all the connectors - and as he did so he noticed that some of the connectors had not been crimped properly and on the outside some of the connectors had allowed water to get in and cause corrosion. After fixing the bad cable, he put new silicon sealers over the outside connectors. Its been about a week and a half and my speeds are stable. The lesson for me is that you can have problems like this without total failure and that it can be fixed with wiring. Everytime before I had problems with the wiring the failure had been total.