Dell Inspiron 5150 - Low CPU speed

Hi,

I've just got back my Inspiron at my office, coming back from Dell support for a motherboard problem.
I've just installed a Gentoo, as it was set up before the motherboard change.
The really ennoying problem is that when I boot, the CPU is detected correctly at boot:

-> cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 15
model : 2
model name : Mobile Intel(R) Pentium(R) 4 CPU 3.06GHz
stepping : 9
cpu MHz : 3059.921
cache size : 512 KB
physical id : 0
siblings : 1
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe cid
bogomips : 5996.54

The problem is that the CPU doesn't run at ~3GHz, 400-500MHz would be more realistic. I've tried a small soft (bogo) that gives me ~900 BogoMIPS. I know that BogoMIPS is not a really relevant measure, but here it shows the gap. For information, I'm running a homemade 2.6.8.1-SMP kernel, but it was the same with the generic 2.4.26 gentoo kernel. I've flashed the BIOS with the A36, in case of...
I don't know what to do, so if anyone has an idea it would be greetly appreciated.

Thank you,
Kerso.