Found a "solution" today, after reading in other posts that an adobe plug in was guilty (adobe air).
I dind't have this "adobe air" plug in on my mac, but I gave it a try with Adobe Flash.
I Deleted Adobe Flash and every flash related plug-in (/Library/Internet Plug-Ins). Hurray !
However, it's a pity that one has to get rid of Flash to get Safari to work properly. Now some videos won't play on my favorite news site.
Note : to show the Library Directory :
Hold down the Option key (Alt) while going in the "Go" menu of the Finder.