2008 mac pro i7 8 core?
There is no genuine Apple Mac Pro 42lb silver tower that uses an i7 processor -- they are all Xeon processors. There are no 8-cores except the Mac Pro 42lb silver tower Mac Pro and the dark cylinder Mac Pro.
Some users of the silver tower Mac Pro have trouble with outdated PCIe cards. A few have trouble with outdated Drivers for those cards and for other things -- be sure all the third-party Drivers and extensions you have are up to date for this version of Mac OS X.
The other possibility is that El Capitan is pushing your graphics card in new and different ways, and your graphics card is buckling under the stress. Screen artifacts are a likely indicator of graphics problems.