Hmmz ... on your end ... if you had installed DD-WRT ... you can adjust the signal rate for the router.
As the Linksys WRT54G (assuming its this model) defaults broadcasts at 82-84db .. pushing it to 100db with the 3rd party firmware is safe.
The linksys highgain +7db antenna isn't of much help especially when you don't have line of sight.
I'm using 2 WRT54G (ver2.2 & 1.1) running DD-WRT ... 2.2 running on WDS mode.
Also do like what the others have suggested, check your surrounding for APs using the same channels then use another not on the same channel and stable.