Sounds like interference. Wireless phones, Bluetooth, microwave ovens .... there are a lot of possible sources.
It's possible to have excellent signal strength and still have a poor signal (signal quality) due to interference or multipath. You could also be experiencing a "null" in the signal; try moving the AP or system an inch or two in different directions and see if the situation changes.
Stick with channel numbers 1, 6, or 11 to minimize your chances of picking up interference.
It might be worthwhile to run Netstumbler (it's free) to see what part of the spectrum is the cleanest for your location.
Good Luck
Scott