Over the past 3 months, I've tried all sorts of trouble-shooting steps I could find including the steps you've mentioned.
The best success I've had so far is to prevent the router from switching between B/G/N, which keeps the connection live the longest.
If I had to take a guess, the iMac is just terrible with wifi routers that allow multiple bands (B/G/N), and auto channel changing.
By the way, I do not have this problem with my iPod Touch, iPad 1, iPad 2, and Windows laptop. I don't seem to have a problem when using bootcamp Windows on the iMac either.