I have an identical problem with an IBM 760. I now have to keep pressure on the plug/cable in order to maintain power supply to the laptop.
I wonder if it is a problem with the cable or the connector.
Must be a reason the 4 pin 'D' is not used on newer models.
(The cable seems vulnerable to strain next to the block of ferrite near the connector).
I have searched for a replacement power supply - but haven't found one yet in UK.
Please let me know the solution.