Maybe it's the size and your system's inability to push that size video across the FSB (front system bus) or memory.
Is the Flash original file a vector movie or video? If a vector animation, it 'renders' the geometry on-the-fly, which takes little bandwidth by comparison, so even an average or slow PC can run it smoothly. If it's video, and it typically is after your conversion to AVI, every pixel is pused through the system memory after decompression (what codec does it use?). You may hit bandwidth problems at the amount of system memory (RAM) or the front system bus (is it a 667 MHz FSB or slower?)
You also may need to adjust settings on your video player. Windows Media Player has options to try some accelerations, if the graphic cards supports it. Try both accelerated and unaccelerated modes. And there's perhaps also some choices for skipping frames in order to stay in synch with 'real time'. If your system is unable to push 24 frames per second at 1024x768 pixels thrlough that system's pipeline, then the player will notice that it's 'behind schedule' and skip some frames in order to catch up, if you have the option selected to do so for the sake of staying in synch. The audio will not suffer, normally, that's the last one they 'skip'. The video is the first one to suffer.
You might also want to make sure there's no other processes running that take 50% or more CPU resources. Use Control-Alt-Del to see and sort by CPU load.
You might also consider re-converting the AVI to another Codec, such as Xvid. What codec did you use so far?
To convert video on a PC try the free tool called VirtualDub, or Debugmode's Wax. If there's no audio involved, then our PD Pro Digital Painter can also handle many video conversions, including image sequences and AVI for which you have the codec properly installed.
What's the operating system you're using, what's the codec in this clip, how long is it, how much RAM do you have? If the player loads the clip into RAM before playing it then there's a chance that you may have 'gone virtual', i.e. run out of physical memory and the OS starts swapping parets in and out of virtual memory (swapfile). That is several orders of magnitudes slower, like 100x slower than in RAM. It can often be a reason for video to stutter as the OS is trying to kep up with chunks of memory from the clip to be taken into RAM for immediate viewing.
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