Thinkpad T43 Power Problem

[macleojw]

Hello,
I've got a ThinkPad T43 2668-46G which has a strange power problem. If it is running on battery or running from the mains without a battery it works perfectly.
However, if it is plugged into the mains and the battery is installed then it switches between battery and ac randomly. There is a also a high pitched buzzing noise which comes and goes. When it is plugged in and it says it is running on battery the battery level does not decrease and if the battery has been discharged it does charge up fine.
When it decides to run on battery when plugged in the battery status in the Power Manager is “discharging (with ac)”.
I've tried different batteries; power supplies and a docking station but it does not make any difference.
The most irritating thing is that the screen flickers when it changes form one power mode to another even if the screen brightness is the same.
I assume the root cause is a fault on the system board. Reinstalling windows doesn’t make any difference so I assume it’s not a software problem. Does anyone have any ideas how I can solve the problem without replacing the motherboard?
Thanks,

James