OFSO - I'm having similar problems with a Belkin N300. My Internet connection is ADSL2 and is nearly always lightning fast. Then suddenly out the blue, once or twice a week, the speed will drop back to the equivalent of a 56K modem.
I've found that the RJ45 connectors seem to be the problem. Some wiggling of the RJ45 connector on the connection to the motherboard, or on the router itself, will fix the problem immediately.
RJ45 connectors seems to be a particularly poor fit between male and female plugs, and a lot of RJ45's seem to often lose signal strength due to poor connections.