OK guys, I think I have put my finger on something very interesting but let me tell you first what I've done this morning :
- Updated my motherboard's BIOS to latest version : as I thought, no incidence on VPX 2835+ ;
- Updated nVidia to yesterday's new drivers : same thing ;
- Activated BIOS multi GPU support and connected my backglass monitor on integrated VGA output : no more smoothness ;
- Ran Scared Stiff with task manager on my backglass : VPX is using a little more than 700 Mb RAM, but no processes takes full control of my hardware.
Here's the interesting part : I have then checked that my directB2S was in exe mode and not in standard mode, as a while ago I was struggling with Attack From Mars that became unplayable because of massive lag due to directb2s in standard mode.
I'm using latest B2S Server by DJRobX and noticed a few new options I didn't notice before : Performance tuning.
Skip solenoids frames is set at 3 by default, I've set it to 1 and voilà, no more ball stuttering, or just a very little juste like in VPX 2830 and below 
You can find the option here :

If I set Skip GI frames to 1, ball stuttering is back.
I've tested with Scared Stiff and Medieval Madness X and with VPX rev 2870 and got the same results : no more ball stuttering with Skip solenoid frames to 1.
I dunno if that makes sense and what are the "relationships" between revs 2835+ and skip solenoid frames, but when set to 1 smoothness is back 
For those who encounter the same thing as me, could you please test and report, just to be sure it's not related to my own hardware ?
Thanks 
EDIT : well, this is more weird than I thought : since I changed Skip solenoid frames to 1 Scared Stiff's spider backglass won't turn around anymore.
I've set value back to 3, I still got no more stuttering but spider won't turn again 
EDIT 2 : well forgot what I've just wrote : rebooted my cab, spider turns again and ball stuttering is back again whatever value I set to Skip solenoid frames 
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Edited by shadowshd, 15 December 2016 - 11:38 AM.