Strange wifi issue iPhone

Ive looked around, and cant seem to find anyone with the exact same problem so I figured Id post.
My wifi works fine, no problems at all.....for about 24 hours probably. Then itll drop wifi and go to 3G/Edge, when I try to reconnect to the wifi it just says unable to join the network XXXXXXX
At this point, I can go into my router, and change any setting...very simple setting...Say from BG-Mixed mode to Wireless-G Only, and wifi will connect no problem first try. 24 hours later, same thign I can just switch it back to BG-Mixed and again, no problems. It happens to my phone daily, my girlfriends every 3-5 days. My wireless computers have not been affected at all. Anyone have any idea what this or anything I can do to fix it?
Also, I did a full restore of my phone last night. Same thing.
Custom Windows 7