1394 packet sizes limited by enhanced speedstep

I found, that I get corrupted or missing data when
listening to a 1394 device when enhanced speedstep
is enabled. Only packet sizes up to about 2 kbytes
seem to work, even though I run at 400 MBits/s.

Is there any way of disabling speedstep other than
doing this in the bios setup?

Udo Lenz, CCD Videometrie