IPHONE 4 DROPS WIFI

I have this problem which has been happening for about a month now.
My iphone 4 will be happily on wifi and then just stops and reverts to 3G only. Going into settings etc and turning wifi off and on etc wont help, it is as if it cant see any wifi at all.
If you reboot the phone it will straight away connect and off you go. But then it drops again and wont for the life of it reconnect. Same for all wifi connections, not just for home or work.. eg at an airport wont find a thing but then you reboot and away it goes..
Same for reset network settings....
Is this just me?
My next thought is to back up the phone and restore a complete factory reset to bare bones and try that...???
Rob
iMac Mac OS X (10.5.6)