I have the same problem over a similar adapter, not sure wich model mine is but it is an Intel Mini-PCI and its wireless G enabled.
Mine isnt based on the time of day though, Mine just enjoys maintaining a connection until you need to use it (Well it seems that way sometimes
)
Sometimes it drops the connection, but according to the Icon in the taskbar it is connected fine, then a couple of minutes later it will finally realise and drop the connection accordingly.
I use the intel utility though so I wouldnt have thought XP could interferre like that?
Problem Dissapears If I use my PCMCIA card, though my machine has a tendancy to crash if I use the PCMCIA while the Mini-PCI is installed (Drivers or not) so it means opening the laptop up to take out the mini-PCI.
EDIT: Just realised the card is on my desk infront of me, mines an Intel PRO/Wireless 2200BG Mini-PCI