These are most often related to noise of the power supply or other transient noise/spikes in the circuit (due to a relay for example).
To be more specific, I'd first remove the relay from the circuit and just use a led instead. If the node works then without problems for longer time, the reason indicates it's a noise issue from the relay section.
Similar examples can be found, e.g. http://forum.arduino.cc/index.php?topic=186879.15
Generally these are solved by separating the power supplies, or 'de-coupling'. A good read illustrating the situation, please read it through: http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
I have bunch of nodes that are troublesome. So fat it's been due to noisy somewhere in the circuit. Also, noticed that some nodes start to malfunction after few years due to low quality capacitors. And that is painful to cure...