Thanks Gaia,
I am quit sure this is not due to Avastar, and if it is the exporter, both native blender collada and avastar exporter have the same problem.
Concerning my connection, I am 15.26 download and 0.87Mbps upload... i think this is just average (?)
The problem comes and goes... maybe but concerning the example i added as a link in my first post, i tried many many times and never obtained a good result...
What is surprising to me, is that i haven't seen any forum talking about this problem ! Are my meshes so bad ?? ^^ Because it happens to me almost all the time... (and that's really disappointing !)