I sat the Prontor SVS on top of the computer overnight (about 38C) and then gave it a few operations.
It got progressively worse and now the lower speeds do not work at all, the shutter sticks open while the escapement fails to spin up.
The escapement can be made to complete by flicking the lever but it is sluggish.

I downloaded the Gauthier service manuals. It appears mine is an early model SVS (10 ap blades) and the manual notes a few design problems with that model in the first year and later came several versions of the synchronizer,
reduced number of ap blades, and a cosmetic change for identification.

For problem that my shutter has per above, the manual says "Rinse mechanism in pure petrol (gasoline)."
The shutter is not lubricated (strict note in manual)
Grease is only used on the outer time ring and the diaphagm mechanism.

I can see some grease starting to appear on one of the shutter blades.
I might send it to the guy who did my Nettar shutter.
He ultrasonically cleaned that one and it works OK now.