Sounds like it may be either channel congestion or perhaps other extraneous interference.
Try changing the channel on your router, experiment try 3 then swap to 11, trial and error.
You may also want to experiment a bit with your encryption (security settings) start by running it open (none) for a bit to test, then WEP, then WPA or WPA 2, you may also want to try changing back and forth between AES and TKIP and see if that makes a difference.