What PC configuration do you have, a lack of graphics memory can be the cause of this kind of problem!
Are you in "high quality" mode when you switch to "shaded mode" view? I don't have solidworks 2012 anymore but it seems to me that this option already existed.
The PC I use is a laptop with an Intel Xeon CPU E3-1545M processor @ 2.90GHz 2.90GHz, a RAM of 16GB, and an NVIDIA quadro M2200 graphics card with 4GB capacity.
When I switch to "shaded mode" display, I tried with a medium quality offered by default and in "high quality", but it didn't change anything.
In the document options, in the "system option" tab, in assembly, the complex assembly mode was active and the "do not show edges in shaded image mode" checkbox was checked.