Arduino Yun - Webcam on Usb Host not fluid picture

Hello, I have followed the project shown on "Alla scoperta di Arduino Yun" by Marco Magagnin edited by Futura Group.
All worked fine apart the problem of wifi discontinuos connection. For that reason I proceeded to SysUpdate 1.4.2 as suggested in this forum.
Now I got a worst problem that is the webcam picture is no longer fluid.
Any issue??
luigidv