IBM Thinkpad R 40 No Boot

Today I have just completed repaired IBM thinkpad R40 laptop that not booting sometimes. Early diagnosis is memory (RAM) was not normal, I was detached then cleansed with pencil eraser, I have re-plug and then the laptop can be turn on normally. But after settling in a few hours the laptop back off again so it will not boot.

Then, I do the following steps:
1. Total release laptop casing
2. Remove the hard disk, CDRoom, cmos battery and memory
3. Re-soldering the legs of memory slots and memory control IC using a blower
4. Replace all the components the laptop and try again
5. The result was satisfactory, normal!

If you find the same problem please try this steps, good luck