How to go about tracking down this error?

Thanks @Gordon, I'll try it again with the next build you post up and see if the problems are gone.

I agree with you on the buffer size, and have now observed the same thing myself.
I'll have a look at the status info and data structures you've mentioned.

I have been doing my own build's in a VM using your guide. One extra step was required from memory because recent releases of Ubuntu omit a package now for VM installs which is where the USB serial device mapping magic happens, I think the package is linux-image-extra-virtual that has to be installed or the USB serial device never shows up as /dev/tty.... So I can continue to play with buffer sizes that way if I need to.

I will look at how node behaves at res.end() and let you know.

Its a shame about the SD file writing occasionally blocking the incoming serial coms. I'd really hoped to come up with a working solution.