The most common cause is that the CPU is overloaded, hits 100% and the video freezes.
Yes, to get any kind of decent video playback quality, you will need to abandon the single core processor. It's obsolete.
You can upgrade a processor, add more memory, "clean" the computer with free downloads, load drivers, or goof around all you want, the video will still freeze, its reality. You can even install a new video card, but it won't do squat.
If you play the video less than full screen, the processor MIGHT have enough space to play a video, but you are reaching. The bigger the playback screen, the more processor space is used and the closer you come to 100% freeze state.
If you eliminate as many applications as possible, (like AOL, Chrome, etc...) you might free up enough processor space to play a video (maybe). But almost nobody is willing to erase junk programs, and therefore will have no processor space left to play a video.
And almost nobody is willing to face up to the fact that single core processors don't have enough cache space to play a video, while running a windows operating system. But the problem is never solved until you face up to the reality.