3 monitor / DisplayPort issue with Radeon HD 5850


Jan 22, 2013

I'm trying to get my system running with 3 monitors, but I can't seem to get a picture on the 3rd one - even though I'm using an active display port -> hdmi adapter.

Currently I have 2 monitors working. These are connected via DVI ports in the 5850.

Now I've still got 1 HDMI and 1 DisplayPort free in the card. I've tried to search the google and several threads, and it seems that I'd have to use the DP port if I want to get a 3rd monitor running. So I bought an active DisplayPort to HDMI adapter from eBay. (Startech DisplayPort to HDMI Active Video and Audio Adapter Converter - DP to HDMI - 1920x1200)

Issue: When I plug the adapter into 5850's displayport, it doesn't recognize the 3rd monitor - but instead I immediately notice some hiccups in the mouse movement, or a slight frame-dropping. Like when I just hover my mouse on the screen, the whole screen seems to stop for a half a second, and then resume moving again. This happens every 1.5-2 seconds when the displayport adapter is connected to the system. I don't even need to have the monitor connected at the other end. I start experiencing frame drops even with just the adapter connected. And then, hiccups stop immediately when I remove the adapter.

I really have no idea as for where the fault could be. Faulty adapter, lacking psu, faulty motherboard, faulty gpu, a driver issue (startech.com doesn't have any, they claim this adapter requires no drivers to be installed),

My system:
Windows 7 64bit (Ultimate)
Gigabyte X58A-UD3R (motherboard)
Intel Core i7 950
Radeon HD 5850
2x HDD
3x AOC 24" monitors (VGA + DVI ports)
450W PSU

Additional devices:
1x DVD-RW drive
1x eLicencer dongle
1x iLok dongle
1x usb stick
1x usb -> MIDI controller adapter
1x usb powered headphones

Would anyone here have an idea or experience with similar issue?