After some research on the net I found examples of people who had solved the problem by calling at Orange (my isp). so that's what I did. It was finally sent me someone who found that my line was unstable and solved the problem. Since everything works nickel and am very happy with this new modem.
By cons it does not explain why before this change the same line unstable worked very well and without any decor with the previous one model a netgear DG834 PN