t400 shut down randomly

[zent]

When my t400 shuts down unexpectedly, it does not show any warning or error, nor beep. It just turns to black screen and everything's off. I suspect that this problem may be caused by overheating. My cpu's temp is about 70C and sometimes when I start the laptop, "fan error" would show up and the laptop could not be started. Another suspicion is the ROM. I use 2 2G samsung DDR3. Although they are from the some company, they are not produced at the same time. I'm worried about this problem. The laptop is old. If it is surely the fan problem, I can invest another 40 bucks on it. If it is not, I guess I should stop wasting money on it since I have already dump over 150 bucks into it. I would like to hear your ideas. Thanks.