DualHead2Go Digital ME + iMac... problem to connect

Hi all!
I have an installation with 2 projector connected to my DualHead. I got not problem with oF app in fullscreen, all works good. But i don’t know why, when i run my iMac, sometimes the projectors see the signal and… very often don’t… and i need to reboot my computer.
I tried with others DualHead but i have the same problem. I don’t know what is wrong. Maybe i have to change the flow turnOnProjector-turnOnMac or maybe there is some stuff to check that i don’t know.
Anyone have some ideas?