I'm not clear as to whether the problem is solved when you restart the router or not. If so it sounds like it could be to do with the IP leasing from your router, especially as it always seems to take the same time before it decides to sulk.
When your wireless connection next goes down, can you go into the command prompt and run the command: ipconfig /renew
and see if any error messages come up?
Also dig around your router settings and see if you can see a lease time specified anywhere? Change it to, say, 2 mins and see if your connection goes again at that new time, then you will know if it is the culprit.
If it is, then a firmware upgrade helps greatly in many of these cases, if a newer one is available. But I'm not familiar with Orange's broadband setup, do they use an off-the-shelf router, or do they customize the firmware like BT do for the homehub?