Dell U2713H Monitor Remaining ON When PC enters Standby/PowerSave mode

I do not sleep or hibernate my computer. However I do have it go into standby turning off both my monitors after 25minutes until I next use mouse/keyboard. The screensaver starts after 15minutes and then 10 minutes later PC goes into standby mode.

Problem I am having is that about once every 3 weeks or so the computer goes into standby and my secondary HP monitor goes into standby but my primary Dell monitor remains on with a grey screen.
Problem is intermittent.

Have a HDMI cable to HP monitor and mini displayport cable to Dell monitor. Recently replaced the displayport cable that the Dell Monitor came with, with a high quality Accell displayport cable that is certified. This did not solve problem.

When problem happens I simply turn off the Dell monitor and then back on again...it then goes into standby as it should as in standby no signal from graphics card. Also after this happens my monitor screens look washed out as if gamma or brightness are excessive. A restart sorts that out.

Am running a Nvidia Asus GTX770 graphic card and other that this occassional glitch all is well with my PC.

Have the latest drivers installed for the Dell monitor.

I would swap the two monitors and see what happens. If the Dell still does not go into standby mode, I would suspect the monitor itself. If the HP now does not go into sleep mode, I would suspect the card.