>>>Obviously my PC is too slow
It is not "obvious" at all, in fact it is nothing to do with your PC.
It is your broadband that is too slow, not your PC.
Every video you watch has to be downloaded to your PC. As soon as you start to watch a video you start to download it. But downloading video can be a slow process as they are so large (in computer terms).
But the problem is you are watching it quicker than it can download.
One option is to "pause" a video as soon as it starts, then let it download for say 30 seconds, THEN start watching it.
Hopefully as it is downloading ahead of you watchng it you wil be ok.
If you still find it pauses then allow more of it to download before starting to watch it.
The only real way to get round this is to get a faster broadband. You dont say what you are paying for, but you are getting less than 1mbps, in fact a third of a mbps (as an example I am on 30mbps - about 90 times faster than yours)