The reason for this is explained here:
http://www.linuxtv.org/pipermail/vdr/2008-October/018072.html
So now we just have to be patient and hope that someone who knows how to fix it fixes it.
I've read this mail but now I've done test recording 47mins long (National Geographic, UPC, 19.2E) and when replaying it (without using any speed change within replaying) the dvb-subtitles were in sync for whole 47 minutes.
So I do not understand why the problems appears only at specific channels.
Are you sure that this recording doesn't have the subtitle "burned in"? What happens when you turn off the dvb-subtitle support in VDR?
Josce