You said the comm rate slowed down. That is at the Comm Server level and has nothing to do with displays. Also given the comm errors, it's pointing to something comm side. Given that the processes are remaining very stable, it doesn't suggest that DmD or the Comm Server are at fault. The only thing left is something in the system dragging butt or the hose to the PLC. Since it works, then slows down until you stop status, and then recovers, it points to something somewhere getting backed up.
When you read large numbers of strings, your packet sizes are going to about 1k. I'm just wondering if the large number of large packets is causing something in the comm pathway to fall behind.
Is the C-more running at the same time? If so, does disconnecting it alter the behavior?