Gentlemen,
I include part, of a system log here; it shows that the crash happens in the module "WindowsServer", in thread 0 which handles and uses the graphics drivers. It is interesting that apple uses "Tesla" Graphics drivers - I'm in doubt that the good oulde 120 card is compatible with Tesla drivers. I'm using a Mac Pro early 2009 with two 120 cards and two 23" Apple Displays connected via Displayport.
Process: WindowServer [381]
Path: /System/Library/Frameworks/CoreGraphics.framework/Versions/A/Resources/WindowSe rver
Identifier: WindowServer
Version: 1
Code Type: X86-64 (Native)
Parent Process: launchd [1]
Responsible: WindowServer [381]
User ID: 88
Date/Time: 2015-10-01 20:09:52.907 +0200
OS Version: Mac OS X 10.11 (15A284)
Report Version: 11
Anonymous UUID: 4D007D2A-71CB-A93C-4FF3-23A453AA5FC9
Time Awake Since Boot: 11000 seconds
System Integrity Protection: enabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Application Specific Information:
abort() called
Application Specific Signatures:
Graphics kernel error: 0x00000002
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff9364c0ae __pthread_kill + 10
1 libsystem_pthread.dylib 0x00007fff8d64f500 pthread_kill + 90
2 libsystem_c.dylib 0x00007fff8891037b abort + 129
3 libGPUSupport.dylib 0x00007fff88ba6907 gpusKillClient + 83
4 com.apple.GeForceTeslaGLDriver 0x00001234402f86e0 0x123440000000 + 3114720
5 com.apple.GeForceTeslaGLDriver 0x00001234402f8403 0x123440000000 + 3113987
6 com.apple.CoreGraphics 0x00007fff96e5b640 CGXGLAccelFinish + 89
7 com.apple.CoreGraphics 0x00007fff96db11be CGXReleaseDisplayDeviceSurface + 227
8 com.apple.CoreGraphics 0x00007fff96e1c52e CGXUpdateDisplay + 14915
9 com.apple.CoreGraphics 0x00007fff96e20c6c update_display_callback + 212
10 com.apple.CoreGraphics 0x00007fff96b94806 run_timer_pass + 547
11 com.apple.CoreGraphics 0x00007fff96e0ee16 CGXRunOneServicesPass + 228
12 com.apple.CoreGraphics 0x00007fff96e111b7 CGXServer + 7174
13 WindowServer 0x000000010322ff7e 0x10322f000 + 3966
14 libdyld.dylib 0x00007fff9a3c45ad start + 1