I'm with orange UK and have the exact same
Problem with my new 4s too!
It will connect but after a short while it stops even though it still shows as connected.
I contacted orange and they did the same old procedure. Restart router, reset network settings, change router chanel etc but nothing has resolved it. In the end they sent me a NetGear router but guess what? I still have the same short term wifi connection!! Never had a prob with my
IPhone 4 that I had previously.