Sleeping mode comme back create problems

Hi, I have a iMac 24" intel. I have two screen, the iMac and a LG. When my iMac come back from sleeping mode, my LG screen lost is resolution. To reset it, i'll deplug the second screen and replug it in back.

When I turn off and turn on my iMac, the LG screen is not detected, i do the same thing, i deplug the cable from back and I replug it.

Very strange.

Someone have an other solution please.

Thank you

François Caron