I have a similar problem with my 3 month old xps8700. I am setup for dual boot. 7 pro and 10 pro. A restart from 10 pro to 7 pro keyboard and mouse go out as 7 pro loads. Shutdown from 10 pro and then startup  to 7 pro it loads normally.. Also same trouble with windows 10 (7) restore cd and active boot disk cd. All is well if starting from shutdown not restart. I have changed to legacy boot and mbr disk to get this far.I am using current bios, anybody using older bios. Thought about downgrading to see if that helps.