This makes absolutely no sense to me
Circuit here: http://imgur.com/mrmPv
@Patduino, dhenry: I thought it was some kind of interference, could you please elaborate more, or perhaps link me to a page where it explains what is going on? I'm not exactly sure what voltage floating is. Also, advice on how to stop this would be appreciated.
... I would recommend a Darlington arrangement which is basically 2 transistors, althogh you can buy power Dalingtons in a single device.See the attached schematic. It will handle a motor that draws up to 10A and is pretty cheap.