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Client speed going up and down could be due to busy network, busy AP, interference between client and AP, and several other factors.
In AirWave, you can do a lookup of the client by mac, see which AP they were associated to. If in VisualRF, you can determine if the AP they associated to was practical for good QOS. From the AP monitor page, you can tell if there's a lot of other users associated to the same AP, or if the AP was undergoing power or channel changes.