why does it worek ok at the Sony shop..

what is dfferent there?

That is the question that needs to be asked.
The picture looks like either moire interference caused by too high of
scan rate for the aperature grill in the picture tube or a video
bandwidth problem.

If it really does work perfect on the test setup at the service shop,
then it stands to reason that it is some part of the equipment that did
not go to the shop. Did the video cable go with the monitor? Did the
computer go with the monitor?

I am guessing that it is an el-cheapo video card problem where the card
has a bandwidth problem or other issue.