Hi,
in my opinion, every information is a useful one and could help others in future! 
About the high PLC load: in some projects you’ll have a high PLC load and have to deal with it even without “doing anythin wrong”.
Therefore, I want to mention 2 more things:
- sometimes it helps to have a look to the task class idle time setting of the project. Reserving a bit more idle time can help to keep the online communication stable even under high load. But be aware, that this reservation also brings even more load on the PLC.
- In the online connection setting, the’re also some parameters available to increase communication timeouts. Increasing this parameters could help to get a (more stable) online connection in case of high CPU load or slow network.
Best regards!