@Krodal:
Don't use ribbon cable for I2C and use two 100nF for the 7805. Every test will be useless until you have this right. I'm serious!
By using a ribbon cable you create some kind of twisted pair for the I2C-bus. The SCL and SDA lines can't handle that. Use two seperate wires.
Can you explain what you mean, please?
I use I2C quite happily over 50cm of ribbon cable carrying signals and power for four sonar modules.