I have no experience burning DVDs, but I can't imagine it's much different from burning CDs. It sounds to me like you're having a problem "dragging to disk" after a reboot is performed. This doesn't surprise me.
I don't know how (or if) a DVD session is closed after dragging to disk and rebooting, but this sounds like the case. You might want to look for some sort of option that allows you to uncheck "close session" when you are finished dragging files to the disk. Hope this helps.