Do the Geckos give you an error for fault finding. Sounds to me like you are getting a following error due to the increased speed and the drives not being able to keep within the following error range. Other thing could be overcurrent as the motor is drawing too much trying to accelerate to the speed.
Try dropping the acceleration and see if that helps, also maybe a bit of tuning is required, not sure how easy or difficult that is with Geckos.
Hood