I have no been able to tell if it's the Arduino, or the drivers in the process. However, monitoring the COM port reveals no output even being seen from the Arduino
May find this a project to build...
http://www.hackster.io/rayburne/the-qbf-signal-generator
I would also consider a totally independent SD-card recorder to save log (debug) info from the second com port on the 2560 (or use Nick Gammon software send only lib).
http://www.hackster.io/rayburne/sd-card-serial-logger
And, put freeRam() on the diagnostic channel just to keep track of weird issues. Source: playground.arduino.cc/Code/AvailableMemory
Ray