using BT infinity in the same room as the hub I get 35MB speeds (wireless) on my IMac but in another room on the same floor it drops to 4MB. Using my v old macbook I get the same speed (35) in the hub room, but it only drops to 20 in the other room.
Looking on the internet it seems that iMac wifi pickup is a recognised issue but I can't see any solutions.
It may be worth install iStumbler (http://www.istumbler.net/ ) or even inSSIDer (http://www.metageek.net/products/inssider/, but I don't think it is free on the Mac) to see if you could get a better channel; or maybe to see if slightly different positions for the hub give stronger signal. Not a solution, but may help a bit.
Please look at my wireless connection help pages below. There are also some other suggestions regarding network connections.
Like you I have an iMac late 2008 model connected via wifi to BT Infinity. In the last few months I upgraded the OS system to OS X 10.8.2 and since then the wifi between the iMac and Hub has been dropping out regularly (like every 60s).Both my MacBook Pro, iPad and iPhone have no such drop out issues and download speeds are a healthy 30-34mb.
I have researched lots of Apple forums and have come to the conclusion that whilst Apple are aware of the issue not much has been done about it. Something to do with the drivers for the wifi having an integral problem.
A few work arounds do exist and can be found on various Apple forums but they dont last long from my experience. So what did I do?
Well as I have an iMac it is safe to say that its situated in permanent position in my office and is not going to move from there. So I thought about running an ethernet connection and by-passing wifi altogether so avoiding the irritating drop outs. However, my master socket/hub/router are situated downstairs and would involve running a cable through walls and under floor boards.......simply not going to happen.
I looked into Power circuit adapter kits like the Devolo 1409 dLAN 200 AV Wireless-N Starter Kit which effectively transfers the broadband signal via the house electrical circuit but I have an RCD fuse box and looking at the reviews of the product the two plugs have to be on the same electrical circuit. I have separate power circuits for upstairs and downstairs so this was not going to work.
In the end I opted for a NETGEAR WN3500RP Universal Dual Band WiFi Range Extender - Wireless network extender with a built in ethernet port. This effectively boosts my wife signal throughout the house and is plugged into an electrical extension lead within the room where my iMac is situated. It was then just a simple case of running a Cat6e ethernet cable from the extender to the back of the iMac.
So, I now have a stable broadband connection in the office, have eliminated wifi drop outs and as its on ethernet it is slightly faster than my wifi experience. Oh, and I didnt have to make swiss cheese out of the house running cables everywhere!