Cisco Wireless :: Sporadic Signal Lost With Linksys WRT54G Routers

I have a office that was converted from a older townhouse.  The building has 3 floors with the distant of each floor expands no more than about 60 - 75 feet from end to end of the building.  Not sure what's in the walls as the building is at least 20-25 years old.  Total number of users is about 20 - 25. 
 
Here is the problem..
 
I have a WRT54G wireless router setup to provide wireless coverage for this office, located on the 2nd floor,  For some reason, about 10 - 15 feet from the opposite end of the building, the signal drops off big time. Regardless of the floor.  So to assist with the signal, I installed a wireless 300N Gain extender on the same floor on the opposite end of the building where the signal was weak.  At first, it seemed to work by boosting the signal, but then it seems to show the same issue as before.. sporatic signal loss, signal fluctuation, etc.
 
My question is should I replace the wireless booster with multiple wireless APs located throughout each floor... maybe 1 per floor?  If so, is there a trick to configuring all 3 APs to have the same SSID and same encryptionsso it will be seamless to the users?  Since the first AP works fine, I wanted to get 2 additional WRT54G units.