I have the same video problem with my sony DSS setup.

I have come to the conclusion that the artifacting is
from the greater amounts of compression DTV has put on
the signal after adding local channels. If you read some
of the other posts on this board, you will see that
others are having similar problems. Hopefully they will
address this problem in the near future.

As far as the audio problem goes, I think you are experiencing some sort of interpretive problem as Mark
suggests. Some receivers DSP functions try and make
mono something it's not!

Good Luck!