Hrm, I wonder if perhaps they just have too many subscribers on that node.... Cable internet is a SHARED technology. Sure it's fast, but if your 'node neighbors' are using alot of bandwidth, and if there are alot of people on that node, it could cause problems... When they say 4 Meg or 6 Meg speeds, that isn't for EVERY customer, because they don't count on EVERYONE using the full bandwidth available at all times....
I would DEFINATLY suggest calling the ISP, and ask to have a Tier 2 look into the issue, and see if there have been other users on the same node, or in the same area complaining of a similar problem. Do you know if any of your neighbors have this trouble too? If you have a spare coax cable, it couldn't hurt to try a different one...