Second monitor via hdmi causing mdp to sleep

Hi

I have a 27" dell monitor which i have been using for a year on my mac mini (2012) via the display port

I have recently added another monitor and im using the HDMI port for that one (hdmi to dvi lead), both monitors work fine BUT then my 27" dell (in the displayport) will all of a sudden go to sleep and nothing i can do will wake it - i have to actually restart and then everything is ok again (until it happens again)

the display settings are set to "never go into sleep" in the control panel and its always the display port monitor, the hdmi monitor is always fine

has anyone had a similar issue, would a usb-hdmi or usb-dvi adapter work better or should i (in theory) be able to run a monitor from the displayport and hdmi simultaniously without the displayport monitor going to sleep

this is driving me crazy!