What I don't understand is why your having trouble with the video cooling now
when you didn't have any trouble in the past with the same cooling.
Usually defective video memory will cause stray pixels like you discribed.
Maybe your video memory is going bad and can't handle as much heat as it could
in the past.
You could try a test. Install coolbits for overclocking. Then under clock just
the memory and test a few games. The memory shouldn't get as hot by doing that
and it should tell you if the memory is part of the problem.

When you mention at start up sometimes seeing these defects is it after Windows
loads or right when you turn the system on? If its right when you turn the
system on then the drivers are not loaded yet for the video card so it would be
hardware related for sure.
The video card does not use the same graphic engine in windows as it does when
your playing games. When your in Windows it uses 2D and in most games it uses
3D.