Pixelated sketch lines

[Isaac SR]

Hi everyone, I´m having problems when I try to sketch over an existing image or used layer, every time I draw over automatically pixelates and I can´t see the last line, and whenever I draw on the outside (blank space) it's perfect. As you can see in the image there are lines crossing over the image and getting this strange glitch and the other lines are visible. I have already uninstalled and installed Ps but in a matter of minutes this glitch appears again... Please help