On the router change the channel for the 2.4mhz to channel 1, 6, or 11. rather than auto.
Also set that channel width to 20hz only.
on the phone go to Settings > Wifi > click the blue arrow beside the network > Select Forget network
go to Settings > general > reset > reset network settings
power the phone off and on. Connect again
For additional assistance in troubleshooting Wifi connection problems go here: http://support.apple.com/kb/TS1398