I2C freeze arduino

Hello,

i'm making a home automation, but for that, i've to communicate between 2 arduino boards.
I've an AtMega328p-pu on a breadboard, and a mega 2560.
The mega 2560 is the "master", it transmit data receive by i2c to usb, and the atMega328 answer when the mega 2560 is asking something.
I use 8m of telephonic cable, with 4 cables (5V, gnd, data, clock) to communicate.

All is working fine, but it freeze at random time, and i don't know why. I tried many libraries, but they all crash after 10 minutes to 4 hours.
Sometime, it's the master, sometime the slave.

I don't know what to do.
Do i have to use an other protocol to communicate ?

Blabla51.

I think is important we see your codes and also a simple image of circuit.
Without I think is hard to find a possible error.

And a protocol is not only how you connect physically hw. A protocol also is software, for example controlling messages sended with a CRC value.

Edit: It was a Serial problem, now it's ok =)