Mavproxy starts up with only garbage packets - restarts fine

I have a number of vehicles I have built which run Raspberry Pi's hooked up via the serial port to a Pixhawk. Mavproxy is configured to start at boot. Some non-zero percentage of time mavproxy starts up in what I call "garbage mode" and just pumps out noise packets - never connects to autopilot. Restarting mavproxy it comes up properly the second time every time. It seems almost like the baud rate isn't configured properly sometimes. I have dug and dug to see if any other process is opening the serial port and mucking with things, but I haven't found anything. Likewise I have played with the startup timing extensively and it makes no difference.

Has anyone experienced this? Any tips? Considering patching mavproxy to have a "restart on excessive noise" function, but that seems like a bandaid.

Begin with MAVProxy's boot-time startup and serial-port/baud-rate handling on a Raspberry Pi connected to Pixhawk. Reproduce the first-start versus restart behavior and trace when noise packets appear; done means the initial launch connects reliably without requiring a restart.

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