system freeze with IBM R40 laptop

[GuanFu Chen]

I have a IBM R40 laptop(which is a celeron 2G,256RAM,20GBytes HDD machine)
About 2 weeks ago I installed ubuntu linux,and it works pretty good at
that time.(In fact,it's the most laptop friendly linux OS I ever seen)
But fews days a ago,the laptop start freeze it self after I use it "EXACTLY" 1 
hour. I can't figure out why. And also can't find what application will freeze
it in an hour. I really need help!