Airtunes iMac->AppleTV kills internet connection on iMac, but not on MBP

My situation. Two Apple computers, one iMac 2.8 Ghz and one MBP 2.16 Ghz, both with 10.5.5, an Apple TV 160GB (with 2.3 software) and Airport Extreme (7.3.2 software). The AE is @ 5Ghz with 802.11n only (I got cordless speakers @ 2.4Ghz, so I chose the 5 Ghz for the AE)

I hooked the iMac to the AppleTV which is in the living room, next to the Airport Extreme. Streaming HD-content on my AppleTV is surprisingly smooth (as it should be imho).

However, when I use AirTunes on AppleTV, after a few minutes the iMac gets disconnected from my AE, leaving me without any internet connection and music from AirTunes. I have to 'disable' airport and then 'enable' it again to get my internet connection working again. However, it will disconnect again a few moments later. However, all the time this is happening, the internet connection on my MBP is working flawless.

This is driving me crazy, to say the least. Here's what I've done so far
- Used the MBP for streaming to airtunes: same problem as with the iMac. The internet connection drops on my MBP, while on the iMac, it's working flawless
- Changed 5 Ghz to any available mode on the AE (802.11n with b/g; 2.4 Ghz).
- No wireless security
- No MAC-address security
- Reset the AE
- Shut down all the interfering appliances (such as my mobile phone and cordless speakers)
- Reinstalled OSX Leopard on my iMac (the MBP is a work-based machine and I refuse to reinstall it for some airtunes problem).

All without any luck. The internet connection dropped twice while writing this post and, like I said, this is really driving me crazy. I haven't always had this problem, so I'm inclined to believe this is a software problem. However, my software knowledge is quite limited: other than the things I tried I have no clue what to do.

So any help is greatly appreciated.