I have been having the same problem recently with mine.
I've got a BT Voyager 2100 Wireless Router with a BT Voyager PCI wireless card in my computer. I'm with Eclipse for broadband internet, not Tiscali.
With mine it will sit there as if connected, but only at 1Mbps not 54Mbps as the router will do, yet when I go to anywhere on the internet everything comes up Server error, and even going to the routers homepage (192.168.1.1) also does the same thing. When looking at the IP properties of the wireless card it comes up with some random IP address starting 169.254.119.106 subnet 255.255.0.0 not the 192.168.1.x subnet 255.255.255.0 it's supposed to have. The other way I can tell when it's gone wrong is by looking at Skype as Skype's icon goes from green (connected) to grey (not connected).
I've also done full checks with both AVG and Norton to make sure there is no spyware/adware anywhere on the computer.
Also sometimes I experience a situation where the laptop can't detect the Wi-Fi signal even when I'm sat waving the wireless box around the laptop.
My only thought to this is that it could possibly be the current heatwave that's interfering with radio waves at present. Having done amatuer radio I know radio waves can be affected by all sorts of things (especially heat), but I wouldn't have expected them to be so severely affected, so much so that a device can't even hear them when sat right next to them.
One other problem we've found in our house as well is that electronic detectors give a lot of false readings in the walls, when we were putting in Sky we tried everywhere in the wall to find out where there wasn't an electric cable and every wall beeped, in the end we just took a chance and drilled, and found out that it was a false reading. My only thought to this is that somewhere in our house we probably have something that's giving off a hell of a lot of electro-magnetic radiation and is affecting all types of electro sensitive objects, the only problem is what figuring out what?? The only thing I can think of which would give off such power is the power station but that's at least 7 miles from where I live (Bewsey in Warrington to Fiddlers Ferry, Warrington/Widnes)
If anyone has an answer to this problem it would be much appreciated.
I have a Toshiba Tecra M3 with an inbuilt wireless card, a DLink router and a Tiscali broadband connection to that.
Pretty frequently the connection goes from 54.0mbps down to 1.0mbps with a few from inbetween. When clicking 'repair' it comes up with 'The wireless adapter could not be disconnected.'
The computer which is connected straight to the router via an ethernet cable also suffers from lost bandwidth - but only when the laptop is also connected to the wireless. At other times it is fine (if you call getting only 512KB/s when you pay for 8MB/s fine, but that's BT's fault).
Sometimes switching the router on and off fixes the problem. Sometimes it doesn't.