buildworld with core2 duo fails with page fault while in kernel mode

Hello,

I upgraded yesterday my mobo from Asus mn2pv-vm + Sempron 2800+ to a brand new
 asus P5W-DH Deluxe + Core Duo 6400.

My freebsd system booted just allright (so it seemed) but when i try to make 
buildworld it fails (always, but after a random time) with (see below, not always with "rm", but similarly) :
I only change my make conf settings to replace CPUTYPE from athlon64 to core2.
Any ideas to help me ?

Thanks in advance

Bruno



Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address   = 0x68
fault code              = supervisor read data, page not present
instruction pointer     = 0x8:0xffffffff80317265
stack pointer           = 0x10:0xffffffffa579c550
frame pointer           = 0x10:0x4
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 8785 (rm)
trap number		= 12
panic: page fault
cpuid = 0
Uptime: 9m31s
Physical memory: 1015 MB
Dumping 184 MB:sr