Inconsistent motor speed

[neic]

Inconsistent motor speed

[neic]

It could be ether the motorcontroller or something mechanical. Try and switch the connection for the two motors.

[neic]

PWM on pin 5 and 6 is not consistent with PWM on other pins.
From Arduino analogWrite() reference:

The PWM outputs generated on pins 5 and 6 will have higher-than-expected duty cycles. This is because of interactions with the millis() and delay() functions, which share the same internal timer used to generate those PWM outputs. This will be noticed mostly on low duty-cycle settings (e.g 0 - 10) and may result in a value of 0 not fully turning off the output on pins 5 and 6.