[LAU] random lockup on shutdown..

I have been trying to troubleshoot this for a couple of weeks now, but have not really got anywhere. I am noticing that my computer locks up on sending the killall5 signal on shutdown, at random times (maybe once every 4 or 5 shutdowns, or maybe even less). After this happens, it will then take ages to start up the next time (taking over 30seconds to get past POST to the grub menu). It seems to me like a hardware problem, but the strange thing is, I have only ever see it happen when using a realtime patched kernel. I know this is not really audio related, but I know there are a lot of gurus on this list! Does anyone have any idea what might be happening? Best wishes, James