I just got a new 39" 1080P HDTV and want to use it as a 3rd monitor. I'm running the latest version of Mountain Lion. I am using the ATI Radeon HD 5770. The card has two mini display ports and one DVI port. On the two min DP ports I have active DVI converters to my normal monitors. The third DVI port I have connected to the TV with a DVI -> VGA cord. The HDTV comes up fine but at a resolution of 1360x768. I know that VGA can do 1920x1080 so I don't understand why it won't work. I had a spare mini DP -> HDMI cable that I then tried on the HDTV and that made it display in 720p. It also killed my 3 monitor setup due to the active converters. (I started to wonder if the TV could do 1080p at all. Plugged in my PS3 and it came up 1080p.) So I don't know what else to try. I considered trying a DVI -> HDMI converter as a last resort. Any other ideas or tricks to make my Mac output 1080p to this HDTV?