I assume by 2-wire you mean RS-485? This sounds like a grounding issue to me (& I have 20 years experience with RS-485). Technically, all electrical circuits require a signal return (aka: the 'third wire' in a 2-wire RS-485). By only using 2 wires, you have 2 'current sources' during transmit, and 2 'current sinks' during receive. Don't let the '+' and '-' labels fool you! Likely there is a grounding or power supply difference between the two sites. Do some research at web sites selling RS-485 isolators - they probably have white-papers on this topic.