Rendered videos showing random frames during playback?

I seem to have a problem when using Sony Vegas pro 11.

My rendered videos will play fine, but then at randoms times, I will see a quick millisecond where it appears as if random frames from other parts of the video have placed themselves in random places in the finished product.

Example, I will be watching the video, and all of a sudden a random frame quickly flashes on the screen as if I have overlapped the video or something?

I have tried setting a brand new project and using default settings, but I find I have to manually delete these frames, which can sometimes ruin the video.

I'd be really grateful if anyone can help me on this. I can also try to make a video to demonstrate the problem if anyone needs it.

I am using Windows 7 64Bit with 16GB of RAM and an AMD Quad Core Processor.