serial to usb converter

[skimask]

Things don't to well with a usb-serial converter, errors everywhere. I am suspecting that the converter may be adding control characters amongst the output but I'm not sure.

I'd doubt it. USB-Serial converters don't (ok, shouldn't) modify the data and add characters as you suggest.
What does the RAW data look like? Do you have any control of that at either end?