Dealing with signal noise over 50 foot communication link
Single-ended signalling is the source of your troubles. Differential signalling will drastically improve crosstalk and noise. RS-422 would be an appropriate choice. 

I2C is not ideal, as its bidirectionality (and, consequently, weakly-driven signals) does not lend itself well to differential signalling. However, it seems you only need one direction. And if you want bidirectional communication, you have extra pairs in your Cat5.