A dedicated long range access point will support these distances: Cisco Aironet 1300s are good for about 30KM.
Sounds like the OP's signal is being de-sensed at the fringes, possibly by a nearby wireless transmission or there's some kind of crosstalk or harmonic image from another signal source interfering with the data stream.
... Running netstumbler on one of the distant clients may help to pinpoint the source of potential interference providing it's not satcom / radar / microwave oven
related.
http://www.netstumbler.com/about/