Can a USB device have a low bandwidth because of poor connection?
What do you mean 'two copies of the black wire'? 

USB uses a voltage differential (3-wire) data bus cable with grounded shield to minimize interference from external sources. Your unexposed wires and unsoldered connections could be a source of noise.

With regard to thee bandwidth error, the macam FAQ mentions lowering image format frame rate. Did you try this?

There is also a bandwidth slider in the video settings dialog which can be adjusted to restrict the USB bandwidth.

You may also find this article of interest, in that it describe a problem with the way Mac OSX hands out USB bandwidth to isochronous devices.

For monitoring performance of your USB ports, the iStats Menu tool offers a range of status statistics including bandwidth performance.