We have quite a few motors on our robot and they all have the same problem. We noticed they seemed weak, so we put a multimeter on them. They’re cycling 5-7-9 volts, never getting to the full 12. However if we bypass the Jags & Victors the motors run much stronger and we measure 12 volts.
All the motors, though? That’s some epicly bad wiring, if so. Sometime you can get effects like this if your code sends different values to a jaguar during a single loop cycle. If you’re accidentally zeroing out the motor command as well as setting it to full on, you can get a weird effect similar to this.