BSOD atikmdag.sys error on 2 x HD6990

The graphics card drivers appear to be corrupted. Please re-install the drivers using the following steps.

1. Download the drivers you want for your display card(s)
2. Click Start Menu
3. Click Control Panel
4. Click Uninstall a program
5. For AMD:
   • Uninstall AMD Catalyst Install Manager if it is listed (this should remove all AMD graphics software and drivers)
   • If AMD Catalyst Install Manager is not listed, use the following method to uninstall the graphics drivers:
     1. Click Start Menu
     2. Right Click My Computer/Computer
     3. Click Manage
     4. Click Device Manager from the list on the left
     5. Expand Display adapters
     6. Do the following for each adapter (in case you have multiple display cards)
        
        • Right click the adapter
        • Click Uninstall (do not click OK in the dialog box that pops up after hitting Uninstall)
        • Put a tick in Delete driver software for this device (if this option is available, otherwise just hit OK) and hit OK
        
        
        Alternatively:
        
        1. Login as an adminstrative user
        2. Click Start Menu
        3. Click Control Panel
        4. Click Hardware and Sound
        5. Click Device Manager (the last link under Devices and Printers)
        6. Expand Display adapters
        7. Do the following for each adapter (in case you have multiple display cards)
           
           • Right click the adapter
           • Click Uninstall (do not click OK in the dialog box that pops up after hitting Uninstall)
           • Put a tick in Delete driver software for this device (if this option is available, otherwise just hit OK) and hit OK
   • Restart your computer after uninstalling drivers for all display cards
   • Install the driver you selected for the display cards once Windows starts

I forgot to mention that I've already reinstalled OS and it did not help Now I have ATI Catalyst 12.3 I have no idea how to cause this error to appear again without waiting another 10-14 days :-/ Thank You anyway.
I hope maybe replacement of memory module will help. We'll see...

You should check the graphics card hardware if software fixes did not resolve the problem. Also, see if using an ATI driver version 11.9 or older helps. The 12.x drivers have caused problems on some systems/graphics cards.

   Warning

Before you proceed with the following, answer these two questions: Are you still under warranty? Does your warranty allow you to open up the machine to check hardware? If you are unsure of the answers to these questions, contact your system manufacturer. WARNING: The steps that follow can void your warranty!!!

• If you are overclocking any hardware, please stop.
  
  
• Monitor temperatures during the following tests.
  
  Use the following programs to monitor the temperatures.
  • Real Temp is a good CPU temperature monitor.
  • Speccy - System Information - Free Download will monitor all hardware temperatures.
  • HWiNFO, HWiNFO32 & HWiNFO64 - Hardware Information and Analysis Tools can be inaccurate for CPU temperatures, but is a good program for GPU temperature monitoring.
  
  
  • Use FurMark: VGA Stress Test, Graphics Card and GPU Stability Test, Burn-in Test, OpenGL Benchmark and GPU Temperature | oZone3D.Net to test the graphics card GPU. Let it run until the GPU temperatures even out or until the GPU temperatures reach a dangerous level (you can find the max temperature for your card on either the nVidia or AMD sites; if you are not sure, ask us). The goal is to get a steady temperature, find out if the GPU is overheating, or to see if there are any artifacts present in the test itself: look for strange pixelated patterns on the screen or colors that do not look like they belong. Then use the |MG| Video Memory Stress Test 1.7.116 Download to test your graphics card memory. Let the memory test run for at least seven passes; the more the better.
    
    
  • Run Hardware - Stress Test With Prime95 to determine any hardware problems. Run all three tests for a few hours each. If you get errors, stop the test and post back here.
    
    
  • Follow the steps for doing CPU stress test using IntelBurnTest
  
  
• Run the boot version of Memtest86+ paying close attention to Parts 2 and 3 of the tutorial. Also, in case Memtest86+ misses anything and comes up with no errors, run the extended version of the Windows Memory Diagnostics Tool for at least five passes. These you may want to run overnight since they take a long time to complete (run them an hour before bed each of the next two nights and check before going to sleep that they are still running).
  
  For Part 3: If You Have Errors: If you swap any memory components, follow these steps for ESD safety:
  
  1. Shut down and turn off your computer.
  2. Unplug all power supplies to the computer (AC Power then battery for laptops, AC power for desktops)
  3. Hold down the power button for 30 seconds to close the circuit and ensure all power drains from components.
  4. Make sure you are grounded by using proper grounding techniques, i.e. work on an anti-static workbench, anti-static desk, or an anti-static pad. Hold something metallic while touching it to the anti-static surface, or use an anti-static wristband to attach to the anti-static material while working.
  
  Once these steps have been followed, it is safe to remove and replace components within your computer.

Thank You. I will do these tests and monitor temp. Later I will try older version of ATI Catalyst ie 11.9 The worst I have to wait after each change (drivers,hardware) until next BSOD. Otherwise I will not know the cause of BSOD The last thing is to replace hardware one by one. Luckly I have another PC working fine with almost the same configuration (warranty is for parts, not for whole PC and I can open it), so it will act as a donor. I will post the result if there is any :)