27 inch Cinema Display Issues: Sound, light sensor

hi,

this is my first post in this forum and i'd like to ask your help.

two weeks ago i got my 27" ACD, which initially worked beautifully.
now i have the following symptoms (appearing randomly):

- sound output through display stops
- sound input through display stops
- onscreen volume indicator appears lagging behind after input (and has no function)
- no auto adjustment of brightness to ambient light
- display mic input incredibly low (compared to built in MBP mic, which is situated about half a meter away).

here is what i tried:

- installing LED Cinema Display Software Update 1.0 (twice): no effect
- resetting SMC: fixes the issue - for about a day
- unplugging ACD power: fixes the issues - for about 4 days
- unplugging ACD USB & MDP: sometimes fixes the issue - four a couple of hours
- deleting the preferences .plist files (com.apple.driver.AppleUSBDisplays.plist; com.apple.audio.DeviceSettings.plist; com.apple.audio.SystemSettings.plist) : no effect

additionally, installing the software update doesn't give me an option to output the sound through MDP, but USB only (see: http://support.apple.com/kb/HT4293).

called apple support, but they have not heard about the issue, have no help to offer, asked me to monitor the issue for another week or so and if re-occuring, i could exchange the display. judging from what i read across the web, replacing the display does not help.

although the issue is being discussed in two other forums, i thought i ask for help here, since this forum seems to be populated by a technically adept crowd.

many thanks in advance all!