Placing images uses the current zoom level and may naturally show quantization issues at anything other than 100%. also placing as smart objects may be misleading due to the SOs having their own logic regarding resolution and rasterization when not used at 100%. Start by checking these things - try to place as a standard layer and make sure you work at 100% or 50% zoom, not crooked values.
No sorry, that is not the problem. I'm working the most in 66% Zoom and before reinstallation it worked.
I delete also all Photoshop Files from my PC and installed the Testversion of PS CS6 - nothing...
Google says nothing - How have I fixed it in the past??? Brain come on!
I'm thankful for futher Infos & Help ;-)
Maybe it's your graphics card driver?
One way to deal with the issue is to do a Select>All before placing a new document (drag an image with the same size into the program window) of the same dimensions.