Cannot restore screen from sleep

I have a dual monitor Ubuntu (15.04) setup, running on an NVS 510 (4 DP outputs), both monitors supporting UHD (3840x2160).

When the monitors go to sleep, and some time passes (it doesn't happen if I leave for 5 minutes, but it happens now almost every time overnight), waking up the computer results in any of the following:
- neither monitor is able to wake up from sleep. I can switch to terminal 1 (CTRL-ALT-F1), waking up the primary monitor, login and restart lightdm - this kills all my apps running, but at least restores to configuration
- the secondary monitor is not able to wake up, the primary monitor is turned on, but after the login screen the resolution is switched to the resolution of the secondary screen, and it is no longer possible to click on things (I think the system registers the clicks in a different position to where the mouse pointer is visible - or because the applications that were positioned on the secondary screen are now moved to the primary screen, windows overlapping each other)
- the secondary monitor is not able to wake up, the primary monitor is working fine
- everything works fine.