FSX CTD with No Error Message

I have been having this problem ever since I bought my new computer about two years ago. The problem only seems to occur if I am flying with an addon aircraft, not with any of the default aircraft, and its not limited to one type. I can be flying along with no problems, then all the instruments will get flashing horizontal lines in them and the cursor arrow also gets these flashing lines distorting the normal arrow symbol, then the screen freezes and eventually shuts down the monitor but the computer still appears to run, there is no error message accompanying the CDT, I have to turn it off manually. When I turn the computer on again it asks if I want to start in Safe mode or Start Windows normally - choosing normally works OK. When I check in Event Viewer under Windows Logs/Application there is no record of the failure ??My computer spec is Intel Extreme X9650 @ 3.00Ghz (unclocked), 4 GB of ram, Vista 64bit Home Premium (Initially) Now Windows 7 Pro x64, Geforce GTX8800, Creative X-Fi Xtreme Audio.I have recently reformatted my Hard drive and installed Windows 7 Professional, updated to all the latest Win7 drivers, I then installed Microsoft Office and FSX, nothing else, I updated FSX to SP1 then SP2 and with default aircraft had no problems (same as before), then I installed the RealAir Duke and after about 5 mins flying I got the above CDT I then installed the new Carenado PA-32R-301 SARATOGA and after about 10 to 15 mins that did the same with the flashing horizontal lines over the instruments and Cursor arrow.Having read practically every article and post on this subject over the last two years I am non the wiser what is causing the problem. I have tried every fix going including going back to various Nvidia drivers, removing one of my graphics cards to improve airflow (temperature is about 65 to 70c now when flying, not excessive). I was hoping the improved fault detection of Windows 7 would find the fault but annoyingly it doesn't. - BTW I have FSX on its own hard drive D:Any ideas appreciatedGlen