Just to update & close this off.
We found that yes, there was an issue with comms but it wasn’t what you’d expect. The ethernet module on the PLC rack in question was being “overloaded” with TCP/IP messages (both ignition & other PLC’s + panelViews in the area).
Fix: Our PLC programmer/electrician installed a 2nd ethernet module and split the traffic (our Ignition stuff, versus process related messages) and the problem was solved.
Apparently the ethernet modules don’t like running above a certain percentage of utilisation constantly and will perform their own “reset” in an attempt to rectify what they see as a “problem” (I can’t remember what that actual value is but I believe it’s quite low - I think around 70% or thereabouts but don’t quote me 
)