It turns out this issue was solved by increasing the Network Buffer sizes under the Buffer Mngmt blocks. The model ran without issue after setting the Buffer pool sizes to: [8196 512 4096 65535]
They were previously set at the default values for the Buffer Mngmt block.
Side note: because this is a concurrent model, with multiple model reference subsystems, there are Ethernet Initialization and Buffer Mngmt blocks within multiple model reference subsystems. To get the model to work, I had to change the pool sizes in all Buffer Mngmt blocks to match the above values.
This is under Matlab R2012b.