I noticed that another user - roofone - seems to have the same problem - described in post 209 of the customiser thread.
Hard to understand why running the customiser without the font change works perfectly - but changing the font results in the usb problem. Oh well, one day the answer will be found. Thanks for giving it your attention. I'll just have to live with the default fonts (which arent that bad).
BTW I also noticed the change to the plain text font commented on by rasbadar.
That's weird too.