I have a similar problem with my current setup. However, the only way I could resolve my problem was by not updating the AMD GPU drivers beyond version 12.3
My rig:-
Windows 7 64bit Ultimate
ASUS M4N68T Mobo
AMD Phenom II x6 1055 Black Edition
4GB Ram ( was 8 but due to dodgy sticks had to reduce it)
ATI Radeon HD7700 GPU
Ok, it is not the best rig in the world but it does me fine and for the most part, it has not let me down yet. Now here is the most important part.
Monitors:-
HANNSG HE195 connected via HDMI to VGA adapter/converter
BENQ G925HDA connected via DVI to VGA adapter
LGTV connected via Displayport to an Active HDMI adapter/converter
Now with the 12.3 drivers which I currently have installed, the GPU picks up the monitors as they should be named. When I install the 12.4 drivers or more recent drivers, the displayport then displays the TV as a "Generic Non-PNP Monitor". This means the resolution decreases to 640*480, but more importantly, I lose the ability to send sound via the displayport to the TV.
In conclusion, I believe that from drivers 12.4 and above, EDID was broken by AMD. I don't know if this has been resolved by AMD yet. I did raise a ticket with them but I never heard anything from them, since the confirmation email of raising a ticket.
The solution to the problem you have here (if it is not fixed yet), is to scrub the AMD drivers from the system and clean install the 12.3 drivers.
P.S. If anyone knows if AMD did resolve this problem, I would very much like to know!