It's pretty much a guess, but you can try putting some stronger pullups on the sda and scl lines.
and how the hell do I do that? lol/ Will that involve other components?
have you tried with the 12v power supply negative not being attached to the negative buss
AARG - 'excellent work'? I was waiting for a pasting because it wasn't a proper electrical diagram. Lol. Thanks.
The 12v thats is connected to the working end of the relays (powering the motors and seems to trigger the error) is a separate plug altogether and not connected in any way to the Arduino.
your drawing shows the 12v negative sharing the negative buss with all the other negatives.
Is there an earth ground in your system?
I think so as the independent 5v and 12v supply are plugged into the mains, and the earths are all connected so I assume yes there is. Actually no. I have just looked at the wall wart plug thing and the earth pin is just plastic, only the live and neutral are conductive. So as far as I can tell, there is no earth ground. (is that what you meant?)
Yes, that is what I meant. An earth ground is a vital component of an EMI suppression strategy.
Oh right. How do you suggest doing that? Having a wire coming from the ground buss and into the ground of a plug socket?
It's a little weird because you don't usually see a ground-only plug. But it would suffice for testing. It also helps if that ground extends to a shield around sensitive components like the controller.For testing, you could also clip the ground to something grounded, like the case of a computer that is plugged in with a grounded cord.