The photos show that the stalling is a fault with the Arduino or its code because the TX does not flash at the moment of stall (the last photo).
You can see from this picture that the Arduino code failed because of what isn't here. OK. Got it.
You've, of course, posted ALL of your Arduino code, and I just missed it. Right?
inStr.reserve(40); // easily enough for longest command (ie.x7000y7000z1404o13s0c19g6i270o14)
In spite of all the warnings about problems with the String class, you are still using it. Here's a gun. Shoot your self in the foot. Go ahead and do it again. There's plenty of bullets, and I'll reload when you run out.