If not using AMBEServer works, then I suspect the serial latency is not set for optimal.

To verify this look at the output of:
cat /sys/bus/usb-serial/devices/ttyUSB0/latency_timer
If the output is 16, that is a problem.

Analog_Bridge sets the for you, AMBEServer does not. If you revert to AMBEServer then before you start AMBEServer:
echo 1 >/sys/bus/usb-serial/devices/ttyUSB0/latency_timer

Then start AMBEServer and retest

I am out in the field today, so I will not have time to work with you on this today.

Steve N4IRS