There are only really two reasons why the video freezes or jitters.
1. Single core processor. You are running so much stuff in your machine that the processor is going 100% over and over again. Reducing the number of software programs installed frees up some processor space and allows your video to play (maybe).
Also reducing the size of the video window, play the video half screen instead of full screen. The smaller the window for video playback, the less processor space will be used, less chance of freezing.
2. Internet provider. The speed and bandwidth of your internet connection is sub standard, and does not live up to the internet providers promises. This especially happens with DSL or dial up. The speed of the data is slower than the speed of the video playback.
Think that upgrading the processor, installing more RAM, or installing a new video card will help this problem?
No, it won't. This is a waste of money. Buy a quad core computer instead, and then enjoy high quality video.
Let's face it, single core processors are obsolete and cannot provide anything close to high quality video.