I'm sorry to hear that you're having this issue. If the video and audio codecs are supported as listed in the manual, then the file should be playing correctly. If no, it may need to be re-encoded to fit the specifications listed in the model.
Unfortunately, I don't have any further suggestions, except maybe trying a different server like Tversity or Twonky for streaming to see if you get a different result.
I have an LN40C550. When streaming H.264 videos from my PC via PC Share Manager, the audio is out of sync and/or skips.
These files play just fine on my PC.
Does anyone have any suggestions?