I've actually got a theory on this; I've had similar issues on my home network, and here's what I think is going on.
I suspect (but have not proven conclusively) that some routers get confused by the DNS Source Port Randomization feature which all major OSes rolled out since 2008, to defend against DNS Cache Poisoning
attacks. The most visible symptom is very slow/erratic web browsing; it looks to me like DNS responses are randomly getting lost or delayed.
My evidence for this is somewhat circumstantial, but it is enough to convince me:
1) Only systems running newer OSes which have been kept current on their patches seem to be affected. Among the motley collection of PCs running here at my house I've got an older Win2K box, and an old Fedora Core 5 box, neither of which have been patched to implement DNS Source Port Randomization. These older systems are the only two which did *not* seem to be affected at all by this issue.
2) Linux systems seem to be more severely affected than Windows systems. (This is consistent with DNS issues, since Linux doesn't normally cache DNS responses locally, whereas Windows normally does. So Linux is making more DNS requests which need to go through the potentially problematic router.)
3) Setting up a local DNS proxy server, and pointing at that for DNS instead of directly at my ISP's DNS servers seems to help somewhat for the Linux systems, reducing the problem to about the level seen on the Windows boxes. (First access to a given site still stalls or times out sometimes, but after that it is smooth for a while... presumably until the entry expires out of the proxy's cache.)
4) Changing the network settings to go through a homebrewed Linux-based router instead of my Netgear WGR614 seems to completely fix the issue. (I've got multiple IPs from my ISP, so it is easy to flip back and forth between the two routers for direct comparison.)
My suggestion to you would be to see if there's a firmware upgrade available for your router. If there isn't one, or it doesn't seem to help, try a different router (or roll your own like I did).Edit
: Looks like you're already planning to try a new router. Good luck, and please let us know if it fixes the problem!
If the world isn't making sense to you, you're either drinking too much or not drinking enough.