Automatic hibernation problems with IBM Thinkpad R400

Hello All,
I have an IBM thinkpad R400. For past few months I have been facing a strange problem with my laptop. It hibernates automatically in 5-10minutes after I switch it on. It happens 8 out of 10 times I open it even after I switch on from hibernate mode also. I tried pulling out my battery out also and tried using my laptop with charger only but then also same issue. So definitely it is not a battery problem. I have even checked the hibernation settings also. But they seem to be fine to me.
If anyone has any other idea please help me out.
Any suggestions are welcome. I am open to all suggestions even if they are pretty straight forward and simple.
I will appreciate your help guys.
Thanks!
Varun

Just to be sure, does the laptop hibernate or crash, i. e. if there are applications open, are they still there when it resumes?

Sorry for the late reply but I am sure that the laptop hibernates and no other applications are open during that time. Please help me if you can. and one more thing, I am sure that it is not a battery problem for sure.
I appreciate your help.
Thanks.