I had this problem a while back and contacted STV who told me they were aware of the problem and were resolving it, which they did.
It has started again recently and when I contacted STV they have said I may be receiving two lots of DTT signals and a rescan should fix it.
Admittedly, it isn't as bad as last time but still gets annoying when it happens a few times in the same programme.
All the other channels, including HD, are fine.
Do I just have to put up with this? 
