Cannot power on my laptop Dell Latitude D600, please help!!?

I currently have the problem with my Dell Latitude D600 that I cannot turn it on. Also, it wasn't the first time I have this issue with this laptop. It happened twice before, the Dell technician had to replace the system board since there is nothing he can do to turn it back on.

Like the previous times, I was able to power on several times (by pressing different Fn keys combination) when the symtoms "can not turn on" started to appear. After few times manage to turn on, it ended up to turn off forever. I had tried many different ways to make it turn back on like plugging power supply without battery, try switching other battery from same model, try switching other functional harddrive, try using other power supply, etc. But there is no help.

I tried to search for this issue in many forums and websites on internet but not really find out the explanation for that. My first feeling is the issue may related to the fact that I frequently put my laptop in hibernate or standby mode without turning it off completly (it saved me a lot of time everytime turn back on, no need to wait many minutes to boot up and reopen all applications that I was working on).

After about 6 months since the 2nd system board replacement, this issue is now back again. My last successful turn on has fallen to the DST (Drive Self Test mode first) and reported a failure like:

"DST Short Results Test:
Test Result: Fail Sad
Error Code: 1000-0142
Msg: Unit 4: Drive Self Test failed. byte = 58"

However I don't think this is the cause of the issue.

The reboot after this check was able me startup Windows as normal. However, I got the feeling that the issue is back again, so I started to backup my data to external hard disk in order to be able to work from other computer. And then, I was right, I couldn't turn it back on anymore, even this time I shut it down completely after use but it didn't help to avoid this issue away. It's now completely deep in the coma!!! :-(

Anyway, I discard the issue related to the power supply since I used other functional power supply but it didn't help, I discard the hard drive issue since I'm able to startup it normally when installing it to my collegue's laptop.

I really don't understand why it happens 3 times. Something else in the laptop hardware has caused the system board failed to response to the action pushing the power button.

Anyone has any idea what's exactly the issue. I don't want another replacement and then this issue reoccurs, I hope my company will replace for me a complete new laptop instead. I feel so sick with this one.

Thanks in advance.
Anthony.