Changed the DVI cable to another one and the issue has pretty much stopped.
Difference between the two cables:
1. All pins and flickers
2. Center 2/3 columns of pins removed= no flicker.
Hazro HZ26WI screen here.
Both cables work perfectly with my MacBook Pro.
I don't understand why it doesn't work properly with one cable yet it does with another and why no other computers have a problem with the cable that the Mac Pro doesn't like.
FYI it affects the GT120, 4870 and i've had three 2009 MPs, three 4870s and two GT120s...