not reaching set speed

We have a BECKMAN J2-HS with a JA-17 rotor SER #: 1756. It cools to temperature, but will not spin to set speed; it will only reach 5000 rpm when it is set to 8000 rpm. Not sure specifically what could be the issue. Looked online and only really says things about potential issues with O rings and/or pins? Please HELP ! Need is ASAP!!