Odd issue - connecting to router

[#1]

Couldn't think of a more suitable title - sorry.
We've got 2 laptops connecting to the wireless router and today it has been acting very odd and I can't make any sense of it
We are now having to run it as totally open as neither laptop will stay connected to the router.
Router was set up using WPA and has been for a couple of years, using different routers same SSID and WPA key. However today neither laptop will stay connected when using the key for more than 5 mins and one has totally given up connecting - and you might know it belongs to SWMBO who is giving me shed loads of grief.
Rebooted both laptops no joy, re-entered the key no change. We are sat in the same room as the router which has been sat in the same place for a good 6 months.
Only by switching to no security have we managed to stay connected.
And to throw more weirdness it is the same on the 2 routers I have, only one plugged in at a time!
There are plenty of other networks around but changing the channel hasn't helped and if it were a interference issue surely turning WPA off wouldn't help, would it?
Thanks for any ideas in advance, sorry about the rambling..

[#2]

Couple of things to try get netstumbler for free from
http://www.netstumbler.com/
See what that makes of the networks in the area as it shows noise among other things. Try at different time of the day to see if someone turns a rooter on at some point, check the channels  they use remember that only channels 1, 5, 9, 13  are clear all other ones overlap each other so so if you are on 4 and there are loads on 3 you may get noise from them.
Its worth keeping a list of other users for the future as if you suddenly loose connection you can see if anyone else has come on.
The next thing is have you bought or changed anything electrical which may effect it? To test for noise get a cheap pocket radio  turn onto medium wave find a place were there is no signal then walk about and see if it picks up any noise if it does you can often trace its source.
When radio network works it sends packets via radio and if it doesnt get to the other end it has to resend it so it slows down or stops working. Encryption increase the amount of work needed to get the right packet so may make things worse turning it off could help

[#3]

You can monitor at the packet level with Wireshark (free) - shows any packets that have to re-transmitted in a simple to use way.
Have you tried moving your laptop(s) nearer to the router - just to see if this makes any difference? 
Try pinging the router (set it on infinite repeat) - does it have phases where it can't find the router?
What does a WEP key do to accessibility?

[#4]

Thanks for the advice... it's corrected itself but I had to change the WPA key. Wonder if SWMBOs laptop had corrupted the key somehow or the router had got confused.
Anyway seems to be ok.. at the moment.

[#5]

Looks like it was the routers wireless dying, it was getting worse range was a few feet and connection kept dropping.
I changed back to the spare router gave it a new SSID and WPA key and all seems to be working again properly.
Thanks for the help again.