Big issue with Flash Player and full screen

Hello (i apologize for my following writing, i'm not good in english... I'll do my best !)

I recently rebuilt my computer, i had Windows XP pro and i am now on Windows 7 (64).

I use Firefox (last upgrade) with the latest version of Flash player ! But now, when i want to see a full screen vidéo (on mégavidéo, WAT or whatever) i got a big freeze ! It changes between the different kind of site i use, but in full screen the videos fall to 4 or 5 fps !!! (when i'm lucky)

First i was thinking for a material problem (graphic card too old or something like that). But 2 things bother me :

1) when i was on XP, i never had this problems at all ! (the vidéos in fullscreen were perfectly fluid)

2) Just in case of, i tried to instal the beta version of Flash Player and active X : 10.1.2.3A and 10.1.2.3P

And, miracle, it's working ! not as well as in "window mode" but i can watch a film with a pretty good fluidity ! 100 times better than the other "official" version of flash !

So the problem is not from my computer, and i dont now what to do !

Why i can't stay in the beta version of flash player ??

Very simple : I have a lot of probleme of ... compatibility ? Exemple : www.deezer.com says i have a too old version of flash, and it doesnt work very well !

And it's the same problème in a lot of other site !

What can I do ?

This is my stuff :

Windows 7 (64)

Fire Fox 3.5.6

Nvidia GeForce 7800 GT

Cpu : Pentium D 2.8GHz dual core

2go DDR2 (Ram)

And this is what i installed :

In my computer :

Adobe Shockwave player 11,5.2.602

Adobe flash player 10 activeX 10.1.2.3A

Adobe flash player Plugin 10.1.2.3P

And in firefox :

Plugin :

-Shockwave Flash 10.1.51.66  (10.1 d51)

-Shockwave for Director 11.5.2.602

And nothing else about Flash ou Adobe ! (I checked everywhere !)

Did i done something wrong ? Did i miss something ?

Please, help me ! I tried to re-install the other version (not beta) of flash a thousand time, but it never works !