Have you done a scan to establish spectrum use and / or interference? Something like NetStumbler (www.netstumbler.com - it's free).
Does the speed shift at a stationary system with clear line-of-sight (i.e., no humans or other critters walking between the AP and client antennas, or even within the cell area ... since it would change the signal pattern).
Basically, it boils down to: If the client is stationary, and so is everything else in the area, and the signal drops, then it's likely to be signal interference and / or you are at the lower threshold of the higher speed and the client or AP is waffling between the rates.
If the client is stationary, but there are moving things (human or non human) within the coverage area, then it's likely to be a Signal Quality issue (SQ is more important, generally, than signal strength). SQ can usually (frequently / occasionally)be improved by re-positioning the AP antennas, or changing to a more suitable type of antenna. These things generally come out with a good site survey.
If you are not imlementing "diversity" (two antennas on each AP in the same coverage area), you probably should be, it's one of the mechanisms used to reduce this kind of effect.
It's usaually a Good Thing to make sure your drivers are current (as possible) .. some of the older drivers, especially Intel and "consumer grade" NICs / drivers, were "a little flakey."
It would be helpful if you could provide some additional details. Wireless stuff is much more complex that it would seem to be at a casual glance. More details are better, generally speaking.
Good Luck
Scott
