The problem is realy the pixels, as I said not doing anything on the Arduino pin 10 where acording to they schematic is the control of backlight, they have normal intensity and off course the trimmmer pot was regulated once and never again touched after.
But as soon and any analogWrite to that pin, the pixels stop having normal brightness, the only way to go back normal is cutting Arduino power and back on again.
EDIT: Sorry... problem solved !!! For some reason after using D10 pin for sending PWM to dim the LCD backlight I was facing that pixels low brightness problem, I just retune the contrast pot and now all his ok in any option. :