Iphone 4 wifi issues connecting to Orange Livebox

Recently purchased iphone 4. I set up home wifi connection with my orange router. WEP key entered, pressed back of livebox and iphone picked up connection no problem. As long as iphone is active, wifi is excellent. As soon as phone goes into standby or is inactive for a while, it drops the wifi signal. I have been told this is a battery saving function and is normal. My problem is.however, once phone is activated again, it forgets WEP key and does not re-connect automatically. I get the message "unable to join.....network". This is not normal. I have had many long and frustrating calls to apple customer care. They all offer different advice and are very sympathetic but so far nothing works. I have got the latest firmware, I have tried restoring phone, forget network settings and many other "solutions". I have spent hours trawling the internet and it appears many others have similar problems but no definitive solution. Apple even provided me with a brand new iphone but exactly the same problems. I cannot fault their customer service but it is still extremely frustrating as I am no nearer an answer. I know my livebox is fine as I have a laptop which connects fine and my son has an iphone 3G which has no problems at all. It must either be an iphone 4 software issue or a compatability issue with the Livebox. Hope somebody out there can help ! I am OK on computers but not fantastic so please feel free to "dumb down" any solutions ( if there are any ) ! Thanks, Mo