Weird issue when iMac connected to Asus display

I have my Asus PB278Q connected to my late 2013 27" imac via mini display port to display port in order to get the maximum 2560x1440 res. it works great however does have one weird issue. Whenever I put my imac to sleep i turn the power off on the display first. I dont use the Asus all the time, but only turn it on when i need the extra screen real estate. When i wake up the imac it behaves erratically. The screen flashes and the cursor moves without touching anything. Anyone know why it does this? Now i also tested putting the imac to sleep with the asus display on and both waked absolutely fine. Its like the imac doesnt know what to do when it wakes from sleep when the asus is off. Anyone have any ideas? Thanks

Anyone have any thoughts? I previously had a Dell U2412 before using the same cable and it never caused the iMac to behave this way. Not sure why it doesn't like the Asus.