This won't help you for the immediate future, but my team bought a few PWM signal generators - sold on Amazon as servo testers - and found them extremely helpful for diagnosing issues like these.
I like this one:
http://www.amazon.com/Andoer-Tester-...dp/B00LTOD4F4/
You can connect several PWM cables at the same time and verify that all the motor controllers are behaving the same.
They're very cheap, but they ship from China so don't expect them to arrive for 2-3 weeks. If you needed something sooner, AndyMark or your local hobby store sells similar devices, or you could roll your own.