iBook G4 wireless issue

I was surfing using my iBook G4 (newest model) using my wireless connection, as normal. My wireless router is in a cupboard upstairs, which has never caused a problem. Signal strength is always fine. However last night I finished surfing, put the book to sleep by closing the lid and then watched some TV. About 30 minutes later I turned it back on and found that it couldn't find the access point. Tried rebooting, shutting down, turning off and on the airport card, and sometimes it found the access point, would try to connect, give an error, then drop the connection.

I had a look at the router, no problems. Tried rebooting the router, no difference. I found that if I opened cupboard door and kept the iBook in the same room I could connect, and the connection would last longer, which to me would suggest that something has gone wrong with either the signal strength of the router, or the receiving ability of the iBook.

I don't have any other wireless devices in the house to test the router with, although I've brought my iBook into work today and connected it to the offices wireless network, which seems to be okay, however there is nothing obstructing the signal in the office, so I don't know if I can compare like with like here.

Any ideas?

Well thanks for the help......

Either way it seems to have sorted itself.