Here's how to determine if heat is the problem.
First, when your computer is stone cold, such as first thing in the morning, turn it on and fly immediately (before running any other programs). If you can fly successfully for a reasonable time, say ten to fifteen minutes, without a problem you have passed step one. Leave the computer on for another half hour or more without running other programs, then try flying again for a reasonable period. If now you run into problems it is almost definitely heat buildup that is the cause. If you do NOT have problems, you probably must look elsewhere, such as some other program that is messing up memory, or something suggested by one of the other contributors to "How To".
During your flight test sessions be sure to give the video a good workout. Make lots of turns, look out side windows, maximize (W) the view, show spot plane views on and off. In other words, don't baby the session -- give it a real test.
The next, longer-term test is to remove the cover(s) from your computer case entirely so as to provide as much ventilation as possible. Under these conditions you ought to be able to fly at any time, regardless of how long the computer has been on or used. This assumes, of course, that the video card has sufficient space around it to enjoy unrestricted air flow and that your case is not enclosed in some kind of cabinet or other restrictive environment.
You now have a choice: Leave the covers off permanently, or install an auxiliary fan to ventilate the case. I have installed one costing $20 that fits into an open expansion slot and blows air out the back of the case. Also, I removed the slot covers from other unused slots so as to permit more air to circulate in and out (this actually might be all you need to do). I positioned the fan so as to be adjacent to the heat sink of the video card. It has worked wonders in my situation -- no more problems. I hope this information might be helpful to you as well.
Happy flying!
Robert Seitz
rseitz1009@aol.com