QUATTRO Motors are two for $7, and I suspect you could buy a lot of replacement motors for the price of a good motor tester/analyzer.  It might be good for larger, higher end motors, but personally I don't think it is worth it for a small brushed motor.  If the motors all seem to spin up and run without any unusual noise, I still think I would make sure I don't have a propeller or gear binding issue of some kind that is causing the instability.  Also make sure that the rotor arms are all straight, and not slightly twisted at an angle.