stray irq13 at runtime

Since updating the i386 package machines the other day, they've all
experienced the following:

May 29 21:24:53 <user.err> gohan28 kernel: stray irq13

irq13: npx0                            2          0
stray irq13                            1          0

This is not appearing during boot - those machines have been up for
hours before the interrupt occurs.

Kris