Noise from switching fans nearby disrupting the I2C bus
According to this source which is an FAQ on I2C bus protocol.

This suggests that typical lengths are 3-4 meters, and with the serial clock running as slow as 500Hz in applications of 100 meters are known to work. 

I suggest you lower the clock speed significantly from the standard 100Khz to something like 8-10Khz or something and do some tests. 

Also, what I2C bus pull-up resistance do you have? With such a long distance, the bus capacitance must be very high so your resistors should be quite low, maybe 1.8-2K Ohm (as shown further down in the FAQ actually!).

EDIT:

May I suggest going away from wires sensors? You can put together a very simple little package that can act as wireless sensor nodes. Then you can place them 80m away if you really wanted to. And you can have lots of them without worrying too much about running cables everywhere. Depending on your level of skill/embedded systems knowledge, you may want something simple like a Synapse like those sold at Sparkfun which supports all sorts of cool mesh networking too if you wanted. Otherwise you can investigate other more in-depth designs...