The problem proably lies with the LT. Are you using a PCMCI card in the LT or the built in? The built in's are poor. You could run a Plenum rated cat5 through the ceiling.
If you convinced you want wireless. Try a USR5450 AP with a USR5410 . I use this setup. 5450 has duel Hi Gain antennas. I'm able to cover my house on lo power. But not near the distance you are doing (50' 4 walls on Lo power).
Go the wired, cheaper. 500' for cat5e plunm, around $60. Then find some one to terminate it for you.
Or mount the AP in the center of the room and allow patrons to use it.