irq19 uhci interrupts taking ~100% of one core?

[Scott Gasch]

Hi,
I'm running freebsd 7.0-RELEASE-p4 on a 4-core amd64 box.  nearly 100% of 1
cpu is constantly being used handling irq19: uhci4 interrupts.  This seems
to happen both with and without any USB devices plugged in:

vmstat -i
interrupt                          total       rate
irq1: atkbd0                           5          0
irq6: fdc0                             1          0
irq17: mskc0 dc0                 1180547         18
irq18: skc0 uhci2*             163250699       2512
irq19: uhci4++                3187989508      49072
irq23: uhci3 ehci1                    31          0
cpu0: timer                    129208570       1988
cpu1: timer                    129208457       1988
cpu2: timer                    125750147       1935
cpu3: timer                    125750122       1935
Total                         3862338087      59452

dmesg
uhci4: <UHCI (generic) USB controller> port 0xa400-0xa41f irq 19 at device
29.1
on pci0
uhci4: [GIANT-LOCKED]
uhci4: [ITHREAD]

Any idea what's going on here and/or how to fix this?

Thx,
Scott