VPI Speed Controller

You say you have a "very old SDS."  Some VERY old VPI speed controllers are not SDSs. So what do you have exactly?

Were it up to me I would prefer an old SDS to the more recent ADS units.

The best controller you can buy for the VPI is sold by SOTA and would include a new motor as well as the Roadrunner for continuous speed measurement.

Other things that you can do to help speed stability is add a heavy peripheral weight as was said, or get the VPI motor-flywheel combo.  

Everything will make a difference.

Stay with your SDS. Enjoy

I had the very old PLC (Power Line Conditioner, the one with the thumb wheel dials) with my TNT-1.
I've upgraded that twice in the last 30 years - first to the PLC with the rotary dials and then to the SDS with the push button adjustment.
If you actually have an old PLC the SDS will be a very worthwhile improvement.
Still have my old TNT with various VPI upgrades. I sold off my Graham Phantom arm and put on a Grace G-1040 arm that I bought for about $150 in 1982. It actually sounds really good!
http://www.tobespages.info/TNT_grace1040._%20small.jpg

Been happy with my SDS.      Something to consider, if you haven't already:     https://www.youtube.com/watch?reload=9&v=AHmYWT3wKqs

Other posts are correct, the SDS is better than the newer ADS.  The ADS is cheaply made and has been the subject of many complaints.

However, you seem to be aiming for high performance.  If you want the very best, SOTA sells a motor-controller-roadrunner combo.for VPI tables.  It has been discussed at length in the VPI forum.

Other ways of improving performance would be the motor-flywheel combo, if they still offer it, and the peripheral weight.