27" Imac makes screen image explode when connecting to HDMI Projector

Recently upgraded from 10.6.8 to 10.8.2 and never had any projector / iMac problem untill now.

When turning the projector on the image explodes (what ever is on the screen, usualy desktop) and the IMac freezes until I remove the projector from the HDMI input. I am using the same mini dvi connector I always used and an Optoma HD20 .

Turning the iMac off plugging it out from the AC and turning everything back up again does make a dual screen possible but not with correct resulotion...

The latest upgrade did not solve this issue.