Is this a new problem? Were things working OK previously? Any new hardware or software?
If this is a new problem try a System Restore to a point prior to the problem:
start > All Programs > Accessories > System Tools > System Restore
Also consider utilizing the warranty.
Does it behave the same on both battery power as well as the AC adapter?
Also:
Look in your Event viewer: rt. click My Computer then click Manage then Event Viewer.
Or: Administrative Tools/Event Viewer or Start/Run/ eventvwr
Open the system as well as the application tabs & look for red
X errors. Click error line for details.
Post back the source & event id & Description especially of errors that seem to coincide with this issue.
Also check the Information lines that might coincide with this issue.
How to view and manage event logs in Event Viewer in Windows XP