The Oxford chipsets are among the best.
Have you tried any other drive?
Dumb question, but you’re not trying to connect your drive to both the USB on the AirPort and another interface like FW400/800 on your Mac at the same time?
If not, I would call Apple and get it swapped out. We have the same device and other than the WD drive, I haven’t had any problems.
It may actually be your AirPort hardware. Apparently, firmware 7.5.1 is shipping on newer base stations and Time Capsules but hasn’t been released to the public by Apple yet.