ath(4) irq and taskq cpu usage

[Eric van Gyzen]

The irq and taskq for my ath(4) card often use excessive amounts of CPU 
time, even when my network is idle.  They are often above 10% and 15%, 
respectively; occasionally, they are as high as 27% and 44%.

The system is an AMD Athlon64 2800+ running FreeBSD 6.2-RELEASE i386 with a 
custom kernel including the wlan_* stuff, ath, ath_hal, and ath_rate_sample. 
  It is a station using WPA2-PSK with AES-CCMP.  The access point is also a 
FreeBSD machine with an ath(4) card.

During periods of high CPU usage, the

     rx failed 'cuz of PHY err
         OFDM timing

fields of the athstats output are increasing rather quickly.  For example, 
while CPU usage was 25% and 46%, the OFDM timing field was increasing by 
43,000 per second.

Can anyone explain this?  Is it a sign of failing hardware?

Thanks,

Eric