Ok, can you log into the modem and navigate to STATUS......CM STATUS. When you are there, please copy the downstream and upstream table data and paste it into this thread. I'd like to have a look at the power levels and signal to noise ratios just to make sure there is nothing wrong with them. This is going to be a process of elimination, step by step. However, keep in mind that none of the combo modem / router devices are known for good wireless performance.
To have a look at your wireless network you can load inSSIDer, which is a wireless network monitoring application. When that is loaded on your laptop, you can check various locations around the house and see how your wireless network stacks up against your neighbours networks in terms of received power. When you look at the network list on the left hand side of the display, you want to see your network at the very top, with all of the other networks down below by at least 15 to 20 dB. Anything less that a 10 db separation and I would expect you to run into problem with co-channel interference.
http://www.techspot.com/downloads/5936-inssider.html