Speed instability is usually caused by a tachometer sesning issue. The tachometer sensor is located on the plate that covers the bottom of the drive motor assembly. They have been known to come loose after years of operation and considering how old this centrifuge is that is definitely a possibility. The other issue is with the magnetic wheel that is affixed to the bottom of the drive spindle assembly. They have also been known to loosen up on the shaft and the magnet that is embedded in the disc has also been known to crack to the point where it starts giving erroneous readings. Finally, your brushes may be wearing out which can result in inconsistent current being applied to the drive motor so those need to be checked too. That's the best I can do from where I'm sitting and feel free to get back in touch with me if you have any other questions about this.