ThinkPad X60s Battery Trouble

I recently bought myself a second hand IBM Thinkpad X60s because I wanted a small laptop to use on the train, which has enough power to run postgresql/mapnik etc. so I can play with maps while I am living out of hotels (better on the body than too much food and beer, but not as pleasant!).
The problem I have is that at every charge cycle the gnome power manager in Ubuntu reports that the battery is less charged than before, and then it shuts down very quickly when running off battery because it thinks it is discharged.
I tried running it to completely empty by letting it suspend on low battery, then waiting for the battery to really die.  The next charge was higher than before, but the same problem has repeated.
I just used gconf-editor to set the action on low battery to 'nothing' so that it will run until the battery is properly dead.   The charge history is shown below:

Basically Ubuntu thinks the battery was dead nearly an hour ago, but it is still running!

I think this must be a problem with ACPI, but don't know how to sort it - a job for a very rainy afternoon.   For now I will just leave the auto-suspend switched off and try to guess when the battery is about to give up.