Any news on this? I am having the exact same issue. Latest version of mediaportal, latest nvidia gfx and have tried LAV, ffdshow, microsoft etc. Seems to happen randomly, but nearly always when i watch a video file then stop it and try to watch another different video file. Sometimes it happens in movies, somethimes videos and even TV. Really dont want to do a clean install again 
If anyone wants to see my log file let me know.
If anyone wants to see my log file let me know.