anyone made a long (1-2m) i2c cable - anything I should worry about? Can use normal stranded wire or should I think about cat5-esque twisted pairs? If so, how do I do that?
I2c analogue devices pdf
Shood take you to a good datasheet
Low C cable with a twist shood be ok for 1 or 2 m
Think cat 5 is about 50pF/m
http://tronixstuff.com/wp-content/uploads/2010/10/p82b715.pdf ?
This is for properly long cables!
Yeah says practical limit without that chip is still a few meters.
Probably got a loose connection then! 
Seems to be about capacitance as @joeatkin2 says. Sound like you’d get a few metres out of 50pF/m cable
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Or the cable has a high C there may be stray C around . Are you using twisted pairs ? If so is one leg of both pairs grounded?
I did use twisted pairs but in a totally uninformed way, just tied each pin to both wires in the pair… Better way?
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That’s not going to work. One leg should be grounded at both ends, so you need two pairs
But a bit of ribbon might do better with a central ground
So I should have clock+gnd pair, data+gnd pair and vcc+gnd pair?
Vcc on its own
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I used 20cm test wires before committing to pcb. A few blips with ic2 timing but worked in the end. Only thing I can suggest is make sure it works with short cable before faulting longer one.
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