I2C bus defective on Mrobotics Pixhawk

I have bought recently, end of January two Pixhawk from Mrobotics.

Unfortunately I got big problems to have teh compass working correctly :http://discuss.ardupilot.org/t/nightmare-compass-calibration-solved/15421/6

I found out that the two Mrobotics Pixhawk boards have a problem with I2C bus .
I have install a Zubax CAN BUS Gps with compass and baro onboard and all the problems disappear .
So it is obvious to me that the reason is that the compass is not anymore on I2C bus.

I tried to add some pull up resistor on the I2C bus with 3,4K but things get worst , without an osciloscope here where I am is pretty impossible to check the I2C bus.

Any suggestions ?