Leave the wireless zero config service running unless you have a third party wireless manager installed as Windows XP requires it to detect and manage wireless network connections. It is not unusual for the signal strength and speed to change slightly, even in the same room as the repeater, but this normally doesn't result in the connection dropping.
I assume it works fine when connecting directly to the uni network? Problems usually show up when something is chaged, and the change is the first thing I'd be pointing fingers at, in your case the repeater.
I used to have a similar problem a while back with a D-Link DWL-900AP+ access point configured as a repeater that was fixed by a firmware update, although interestingly this didn't help in your situation. What model of AP is it? Do you have another unit you could try in its place?