Mavlink Errors Over Serial Com

This is a bit of repost of a question I asked in the Gitter Firmware channel. I’m wondering if there are some who frequent these boards but not the Gitter might have some insight.

I’m testing a connection with Mavlink over a FTDI/TTY cable on Telem2 and the 3DR radios on Telem1 (alternately, to keep things isolated) I’m also on the master branch of the firmware instead of my own, again to keep conditions isolated. Both connections work with QGC just great, but each one hangs at a different spot on the two test/example files, mavlink_shell.py and c_uart_interface. When using the ftdi cable and running ./mavlink_control -d /dev/tty.usbserial-FT0DJXDB I get the following output :
> OPEN PORT

Connected to /dev/tty.usbserial-FT0DJXDB with 57600 baud, 8 data bits, no parity, 1 stop bit (8N1)
START READ THREAD 
CHECK FOR MESSAGES
Found
GOT VEHICLE SYSTEM ID: 1
GOT AUTOPILOT COMPONENT ID: 1

Which is where it hangs.

Running the same program over the 3DR radios, the program runs this much further:

INITIAL POSITION XYZ = [ 0.0000 , 0.0000 , 6.7058 ]
INITIAL POSITION YAW = 0.8231

START WRITE THREAD

ENABLE OFFBOARD MODE

and then hangs.

I can post up the results of running the python shell if that would help.

I’d like to establish a cogent connection before I start writing my own code on this to keep a legitimate baseline of expected returns.

Ideas for debugging this are greatly appreciated!