This Sequential Circuits Prophet VS came to me with a broken data slider and some intermittent fault where the system would just stop running after some while, as like staying in reset-like state after warming up.
Although I could not reproduce the above described fault I found that the power regulators on both boards got really hot (burning your fingertips at touch) so I concluded that this could lead to that kind of system reset. The main pcb even showed some brown discolouration from the excess of heat.
I decided to replace the regulators with new ones and to redo the thermal junction between regulators and radiator.
On the voice boards, I added as well some little radiators to the power regulators in order to get rig of excessive heat. Also, the tantalum caps of the power supply have been replaced during that same operation.
The defective dataslider has been replaced by a 10k lin Alps stereo slider with exactly the same dimensions as the original one.
So far the owner of the synth has not experienced that reset problem again, so it seems my guess on the power supply was right.