First of all, is the PC you use with your Fantom G used exclusively for your music media or is it an everyday PC that is loaded with other software? If it is the latter then something could be conflicting with the Fantom G driver and or the Editor software itself. A simple fix that "might" work (I am assuming you are using Windows XP or Vista) would be to use System Restore on your PC and restore the PC to a time "prior" to when all this started happening. Of course, doing a system restore can affect other software when you restore the PC, so sometimes that can be a hassle having to update your other previous software that gets affected when you do the system restore process - if any actually does. OTOH it could, as you say, be the Fantom G itself i.e. hardware issues on the Fantom G. I'm surprised we're still getting people who have "volume spike" and distortion issues with their Fantom G. If you purchased your Fantom G recently it shouldn't be affected in this way I don't think. But if you purchased early on when the Fantom G first came out then I could see a possible correlation.
If you know for a fact that your USB cable is good then the problem lies with either the Fantom G or with the PC side of things. I hope you get it straightened out okay and soon. PS: You might want to call Roland Tech and see if they can be of some assistance if need be too.