Start with verifying the level of the machine. Next adjust the gibbs and straaps in all three axis. Next verify the thrust bearing slop and ball nut to ball screw slop. Verify that your servo loop gain is adjusted for 595 because you have DC servos.
If you still have the problem start checking the tachometer in the motors, the resolvers, the possibility of a weak motor or a weak amp.
Since you have replaced the couplers, verify that the two sides of the couples are not bottomming out as they rotate.
This is a lot of checking but this should help you find the issue.