Robert,
Underlying protocol is TCP and it is a combination of Gigabit fibre + twisted pair.
Not sure of your definition of “a lot of activity on the network” but I’d say it was fairly busy yet not too busy for existing standard PLC control traffic. The area has its own VLAN with redundant gigabit fibre between HP Procurve gigabit switches. The Ignition server resides on a separate VLAN & traverses a firewall to get to the PLC.
I did have a close look at the Console event logs & found an instance where the error occurred. ther was an error shortly after the initial one that said something like response found when not requested or something along those lines (I should have taken notes 
) so you may have a point with the “bit is getting dropped in the comms path”. Any idea how I can “stretch” the wait time for the PLC responses? Or am I barking up the wrong tree here?
Peter