i am also having the same problem with my 785
usually it will pause at 3% then get to 25% then reboot then it will slowly amble to 60% shoot up to 80% then reboot and crash requiring a safe mode firmware flash then try to upload again
at one time a week or so ago it took me over 2 hours of this to eventually get it updated
this included trying it on all 3 of my pcs in all usb ports
it is getting to the stage now where i am reluctant to update any setings and am resorting to unpacking my original remotes which sort of defeats the object of having a harmony
i never had this many problems with my pronto's