Recoding RX/TX PCBA Firmware

I have a matched RX-TX PCBA pair that currently runs on buggy firmware. My main objective is to clean up the code base so the boards communicate flawlessly again; every effort should go toward identifying and eliminating the faults that break or destabilise the link... (Budget: $30 - $250 CAD, Jobs: Arduino, C Programming, C# Programming, C++ Programming, Debugging, Electrical Engineering, Electronics, Embedded C++, Firmware Development, Microcontroller)