I'm sorry, you did post the sketch. But I'm still missing the link to the SabertoothSimplified library, although I don't think that it is relevant, I just want to check it.
Try installing pull-up resistors (I'd start with 4k7 or 3k3) because on the MPU6050 (given you use the Sparkfun module) are only 10ks installed and with some instability from the driver this may cause errors on the I2C. If that doesn't help, get a bi-directional logic level converter (one that is made for I2C, p.e. https://www.sparkfun.com/products/12009) between your MPU module and the Arduino.