Increase the CIP connection size to 4000 as well. You may want to try various connection sizes again after changing this.
Ultimately, though, it's as Phil said... you are requesting more data at faster rates than the PLC is able to serve up. Some of this may be due how tags are being read - if you are using AOIs the Ignition driver cannot read those efficiently. (many more details available in the linked material).
Go to the PLC's status page in a browser and find the Tasks tab and look at the comms usage.
