Fixing recursive fault but reboot is needed

[sourcemaker]

I have a urgent problem related to the ubuntu kernel!!!
My system crashs every day repeatedly with the kernel message:

"Fixing recursive fault but reboot is needed!"


My main problem now is, that I do not know the reason 
for this panic and I also can't reproduce it. 
Sometimes the systems runs 4 hours and longer until 
the panic happens. On the other hand the systems crashs
short time later after reboot.


Ubuntu-Version:
9.04

Kernel-Version:
2.6.28-3-rt #12-Ubunt