Mobile Pentium III 750/Speedstep in IBM TP 600x

I bought a Mobile Pentium III 750/Speedstep on a MMC2-Module (PMM75002201QS) for my IBM ThinkPad 600X (Mod. 3645-8EU) originally equiped with a Mobile P3-500 MMC2.

There is also a version of the TP600X that comes with a Mobile P3-650/Speedstep - so the BIOS should be able to handle Speedstep-CPUs, the termal power of the 750MHz-CPU is at about 20W, the 650MHz has 18W and the 500MHz-Model 15W!

I made a BIOS-update to the latest level and changed the CPU but I have troubles with it :-(

I can switch on the ThinkPad but often it turns off some seconds later, sometime I have enough time to enter the BIOS or seeing the Bootmanager, sometimes I even can start my OS (OS/2 Warp 4.52 or Win98se) but after some time the ThinkPad switchs off :-(

The BIOS recognises the CPU correctly as iP3 750MHz with Speedstep, the speed of the new CPU is as high as expectect and it don't seems to be a temperature-problem - everything stays cold as before and even after long time and first switch on of the cold ThinkPad it happens that it goes off directly or later. I measured the current-consumption of the ThinkPad with both CPUs and it is some times even a little bit LOWER with the new CPU or at the same level!

So theoreticly the CPU should work in this computer, but in practice it doesn't - has anybody in the group any idea what the problem could be and how to solve it?
Is it possible to "underclock" the MMC2-Module to 650 or 700MHz to test it?

Thanks a lot for your answer,

Thorolf