I've got a late `09 27"iMac with internet issues.
I've read through this entire string and tried most everything.
Things I know: my wifi connectivity seems just fine. Changing the DNS numbers seems to work...for awhile. Changing the wifi router channel seems to work... for awhile. By awhile, I mean a couple of hours of usage.
I've eliminated extraneaus bandwidth issues like "baby monitor", competeing wifi signals/channels, cordless phones, cell phones, and external hard-drive back ups (USB).
The WiFi seems fine. Every other device works fine with my WiFi router & has great response, even when using many at the same time.
My issues did not show up until I advanced to Lion and bought/used an external HD for back up.
Going back in Time Machine worked...for awhile.
The issues are strictly with internet usage: iTunes, AppStore, Google Earth and web browsers (all of them). Speed with all other applications is superb and as expected (All Adobe products, Strata, Anime).
Theories
1 - It's some kind of memory issue.
It seems to compile over time and usage. It just "locks up" after a couple of hours or minutes depending on what I'm doing. The issue HATES YouTube and Downloading iTunes Podcasts while doing ANYTHING else on the internet. Which leads me to think it's...
2 - Malware/Virus. But nothing shows up on any (many) scans.
or
3 - An Overheating issue. Some component just hates this much activity. I don't know what Lion does but I'm pretty certain it doesn't cut down on processes. From what I've read, it doesn't seem to be the WiFi unit in the iMac itself (cuz folks have replaced it), but something it's attached to. This leads me to think its a motherboard problem. The unit runs pretty hot. Apple's lack of attention on this (I'm now realizing) prevalent issue would mean a lot of (expensive) recalls. I just want to surf when I want to.
I am going to try aiming an external fan at the back of my iMac. Without input from Apple, I am at a loss as to what else to do.