I have a VSI and associated capacity flask, that I use for demonstration purposes. If I hold it near the ground, then raise it above my head, it will read +100 fpm, then slowly drop to zero. Lowering it again to floor level shows -100 fpm.
This sounds very interessting.
I wonder how fast you actually moved it? 100 fps is equiavalent to 0.5m/s i.e. approx 4 seconds from floor to over your head. My guess is that you are moving it much faster? Or is it not the instrument you are moving ?
What you probably have is an IVSI, that contains accelerometers which forses "false" pressure into the system when accelerated.
Try moving it slowly instead (like the 4 seconds from floor to top) and see if it actually displays 100 fpm.
The fact that is "slowly" drops to zero is the very lagging behaviour that causes som of the problems. If it where a precise instrument, it should show zero the moment you stop the movement.