Wireless will "Downshift" if the SNR (Signal to Noise Ratio) changes. Kind of like your car downshifting when you're going up a hill (more resistance).
You could try changing the wireless channel, and see if that helps. Using a tool like NetStumbler will tell you if there is other wireless in the area on the same channel that could be interfering.
If you *do* find other wireless frequencies in the area, make sure you set your channel at least 3 channels away... wireless channels "Bleed" a little bit into adjacent frequencies.
Another thing to consider is stuff in the 2.4/5 GHz range. Microwaves, portable phones both can cause interference. I was working in the break room one day when someone started the microwave, and I lost signal completely.
Shifting speeds is a normal occurance, however. Unless it's impacting what you're doing, I wouldn't worry about it.
Oh... and don't trust the signal bars on the system tray. Better to use something that actually shows SNR, like Stumbler.
Just my 2¢
"What the captain doesn't realize is that we've secretly replaced his Dilithium Crystals with new Folger's Crystals."
--Greg