The observations he sees are the same I see when running the faster baud rate. I get a bunch of PKT_RDY packets with a few speech packets sprinkled in. The thread mentions something about the TI TPS380x-Q1 Voltage Detectors cause this hardware reset of the AMBE chip. Has anyone else run into this issue?

This indicates a hardware issue.  If there were a hardware issue, we'd have tons of folks complaining about bad hardware.  We have been selling the ThumbDV at 460800 for years now with no issues on Mac, Windows and Linux.