... and since you have a celeron the problem is worse, celeron is not a good cpu, is the low end of cpus...
True, but don't diss Celerons so easily.
The latest versions of Celerons (the Exxx series) has surprisingly good HD video capabilites, even with low-end video cards.
For example, I tested a Celeron E5200 (2.6 GHz Dual Core) in a Biostar G31 motherboard, with the integrated Intel video card.
It plays HD Youtube videos at 1080i in full screen without a glitch. I couldn't believe it, but I saw it myself.
In fact, I believe that these Exxx Celerons are in fact degraded Pentiums; I know that Intel does that, so it's possible.
The only problem is that it plays HD youtubes perfectly in Windows - sadly, not so much in Linux.
In PCLOS it happens just what OP says; the youtube video is jerky, but the downloaded video plays perfectly using SMPlayer.
If I can, I'll try reverting to an older version of Flash, as suggested; just to see what happens.