If this were a firmware/driver problem then wouldn't the problem exist regardless of the playback software? The fact that vdr has playback sync problems whereas mplayer doesn't when playing back the same file suggests it's a vdr problem... But this has already been covered.
Afaics it is a firmware/driver problem. It should be fixed there.
Werner is currently working on a modified A/V sync code.
We will see whether is improves things. Please be patient.
Oliver