Mega + Protoshield + DS1307 module = Nothing...


first of all excuse me for my bad English.

I have an Arduino Mega (last version) and the official mega protoshield. I have a ds1307 module that works fine with I2c.

I use 4 wire to rely arduino and clock module :
1 Vcc 5v

when i plug directly VCC to 5v, GNG to GND, SDA to SDA and SCL to SCL on Arduino it works fine, serial monitor display a line every second with correct hour and date.

But when i am using the protoshield, it won't work. The serial monitor display nothing.
I have searched for hours, it seems that when i plug SCL and SDA on protoshield, the serial monitor stops and when i unplug it, it sends lines...

Where is the problem ? Any ideas ?

Thanks a lot