[HELP] Dazzed and confused / machine reboots unexpectedly


I have reboots very often about every 2-4 hours.
Can someone help me where to start debugging?

It operates perfectly fine when it works, and then suddenly reboots.
I only have some IVR for E1 lines, no IP no user extensions, only information is given on the IVR using AGI, buttons pressed saved to mysql. Completely stucked as I do not se any sign prior.

Kernel: 2.6.15-1 stable
Asterisk: 1.2.4 stable
Zaptel: 1.2.4
Libpri: 1.2.2
HW: TE411P Digium card in IBM xSeries336, 2x3.2G XEON, 2G RAM

kernel: [17272708.804000] Uhhuh. NMI received for unknown reason 35 on CPU 0.
kernel: [17272708.804000] Dazed and confused, but trying to continue

Any input are most welcome,
Ferenc Fertoi