I'm having problems with the wireless network in my flat. Basically, we have 5 PCs on one floor, and 4 on the floor above, all sharing one 8mb connection.

Currently we have a network set up as follows:
The access points and repeater are these: http://www.ebuyer.com/UK/product/112497/rb/24792546253

We followed the Gentoo Home Router Guide to set up the router, found here: http://www.gentoo.org/doc/en/home-router-howto.xml

We've had the network set up for around 2 weeks, and for the most part it works fine downstairs.
The repeater upstairs gets a good signal from the access point downstairs, but the connection frequently drops completely. Computers upstairs can recieve an IP from the DHCP server on the router, but then often are completely disconnected after sending/receiving a few KBs.

Whilst we have had some problems with DHCP, and IP address conflicts, but the frequent disconnections have only really been a problem for the past day.

We have tried swapping the access points and repeater to try and find a hardware fault, but this made no difference. We have also restarted all hardware.
The problem exists upstairs even when we prevent all the downstairs computers from connecting.
All the computers upstairs are running Windows XP bar mine which is running Gentoo, but all seem to suffer the same problem.
We filter access to the network by MAC address on the first access point/repeater rather than the router, but we have disabled this to ensure it wasn't what was causing the problems.

If you want more info on anything, or any outputs of logs or contents of files on the router just ask, I don't really know what would be relevant to post. We don't have any sort of traffic filtering on the router... but as I said we tried connecting with the downstairs network disabled, and still had problems whether we were trying to download a specific file or load webpages with only one computer connected upstairs.

Any help or advice would be greatly appretiated.

Nice diagram.
You wrote that you tried "...swapping the access points and repeater...", your diagram only shows 1 repeater, did you have a spare to swap out?
Check the signal strengths to the upstairs systems, sounds like the signal is either weak or fluctuating due to some sort of interference.

No, the repeater is the same hardware as the access points, just set up differently. We swapped the positions of these to check each one worked, and we had the same problem with each.
The signal we're getting upstairs is as strong as most of downstairs, but none of the computers downstairs are experiencing this problem.

So the single to upstairs systems are good, excellent.
In the diagram you show connection between the repeater and AP with booster antenna, is that AP also downstairs?

Are the AP's broadcasting same SSID? If not, try different SSID on one of them, then check the signal strengths.

Perhaps I'm going down wrong troubleshooting path, but just seems to be a wireless signal problem.

The second AP is also downstairs.

We have a seperate ssid upstairs, downstairs, and between the AP and repeater (because that's how the software seems to like it).

Thanks.