Tom how about rebooting the host after an hour to refresh the UltraStudio?
We have one here but dont have a 4K display yet (tested a few).
With FCPX I dont have that problem using a Flanders HD, Sony PVM HD and NEC HD LCD at the same time even after hours of use.
However I tend reboot it anyways just in case.
I still cant get Premiere to work using the UltraStudio 4K and with Avid we get the annoying BOB error but it at least works.