E220s freezing without symptoms after renewing fan and thermal pad

Hello,

I recently renewed a rattling fan in my E220s. Since then I get permanent freezes every time, starting from ca. 5 to 40 minutes on. I checked with Sandra, and to my understanding no temperature sensor shows anything abnormal.

Since the thermal pad on the chip to the left from the CPU (northbridge?) seemed worn, I renewed it also with a 1 mm 7W pad. After dismantling again it does seem not the whole (but no less than 80%) of the chip's surface is hard pressed to the pad. I understand it's all the screws that keep the structure tight, but I have no more ideas on how it can be wrongly fit - or improved. Please note I have dismantled the laptop including the fan assembly several times before, without any noticeable issues. Yes I do use thermal grease/spread for the CPU.

Many thanks in advance,

Lenovo ThinkPad Edge E220s