Tested two N900's and two cables and plugs. I am not sure what is the root cause of the problem, but I assume only having 4 small pins making contact is not enough and is adding resistance.
There is no noticeable loss of contact during charging - the contact itself feels pretty firm (that is also the reason why I was so positive about it at first).

It does charge sufficient when the device is powered off but I need to have it charged while using VNC and wifi. No problems like this using a micro-usb cable or adaptor.