Wow ! after 2 days i found the problem, the Mega board have 2 hardware resistors pulled to +5v , therefore is impossible to disable it by software, you can see in the schematic here http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf in the upper right.
On the board the resistors is a four-pack right to the On led.
This has caused several problems, because I disabled the pull-up in Wire.h library(without effect), and pull up I2c with external resitors to +3v3, but with the unknowed resistors to +5v I added serial resistors from 5v to 3v3 line via SDA/SCL, therefore extra volts to the 3v3 line(4,5v mesured) , and generate unstability.
Caution with Mega board an 3v3 devices, is easy to have tosted....
I hope this help to others.
Thanks