DELL Latitude D620 Power problem

I have a Dell Latitude D620 which alternates itself between AC and DC power for a split second with no reason.  This results in the screen going darker for a split second as it switches to battery power, this happens a few times every couple of minutes.  If you go into Control Panel in XP and look at the power, it changes for a split second from the power supply to the battery!

 

I have tried a different power supply and a different battery with no resolution.  The ambient light settings are disabled. The laptop does however seem to work in a docking station.

 

Has anyone else had or resolved this problem?