It's been a while, but in case anybody stumbles on this again: looks like a bug with this particular mainboard (ACPI implementation?). See here:
http://forums.freebsd.org/archive/index.php/t-939.htmlThe workaround proposed in that thread could be applied to m0n0wall by adding the following tag in the <system> section of config.xml:
<shellcmd>sysctl kern.timecounter.hardware=TSC</shellcmd>