There is an undocumented "soft start" function on the SewQuiet 4000 and SewQuiet 5000. This is the "S.1" function, which is show in this graphic:
I have my S.1 set at "1", which makes it a little less jumpy.
I have not done the modification originally found on leatherworkers.net
(which has now been updated to use stacked post-it notes for optical density instead of pencil lines.) My warranty is up soon, so this may be a weekend project.
The modification makes the control more "linear", rather than the almost logarithmic ramp-up of the stock unit. Read about it here:
Watch a very thorough video by the same guy who did the above write-up, including a demonstration of the smooth ramp-up before/after mod here:
Servo Motor: DIY Smoother Speed Control MOD - YouTube
Watch a video comparing the slow speed control of a SewQuiet 5000 to a clutch motor here:
SewQuiet 5000 vs. a clutch motor « Sewing Dude