This is the third maybe forth time I have centered the NIS, and then again, a long way from home, turn key nothing,take cover off center NIS, make it home.
A little blue locktite maybe... Sounds like the spring or nylock (not sure which yours uses) may not be resisting the adjustment moving due to vibration.
Stantard locktite warnings... use sparingly, keep off of plastic. Don't get it anywhere but on screw threads.
Note that this solution requires up to 24 hr from application until it works. (usually starts helping resist vibration moving a set screw in about an hour, but its not set up fully for 24 hr)
I like cheap solutions. If a small drop of locktite solves it then that's a win.
