Playing Videos

It's a processor problem. The processor does not have enough cache memory to handle all of the applications that are running. This happens especially with single core processors.

You can eliminate as many applications as possible to free up the processor for video. you can add RAM, put in a better video card, all of which will produce very little or nothing.
when you push Ctrl Alt Delete, you can watch your processor space increase / decrease according to the number of apps that are running, in the Task Manager.

Now as you notice, the bigger the video window, the more processor space is being used...until you reach 100%...at which point the video fails, freezes, jiggles, etc...you have run out of processor space.
in order to get problem free video at full screen you need enough space, so that the processor never reaches 100%
and that's what quad cores were made for.