@Mark. I have set up RTS flow control. I am not sure if you are talking about something different. I always operate 8N1 for both the 230400 and 480600 and only the slower works. 
@Bob. The thumbDV is running with a linux 2.6.37 os and I do not believe there are any other COM port drivers.

I was able to find this old thread:
https://dvswitch.groups.io/g/main/topic/5521534#315
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?