A proper site survey will tell you all you need to know. There are several things that could be causing this. Interference and many other forms of RF Lan degradation. There are several tools to use to gather most of the channel, frequency, and range information of any potentially interfering wireless lans. Being able to decipher the information is another story all together. A few simple things to make note of....
Are all clients experiencing this drop off or just some. If all clients except for one or two (which might be located closest to the access point) then you are probably experiencing a specific type of interference known as "near/far". If all clients are experiencing the drops, then it's more than likely either multipath or an external source (other lans, or other RF interference sources).
Did anyone do a site survey prior to implementing this wireless solution?
CISSP,ISC2 Affiliate & Instructor, MCT, MCSE2K/2K3, MCSA, CEH, Security+, Network+, CTT+, A+