im new-ish to banjos,
i have an openback that iv had for a month, and noticed that while i was playing, the strings would 'warp' in and out of tune.
like the effect of a whammy-bar on a guitar...
i discovered that simply, the neck nut against the fret end of the pot on the adjusting rod wasn't tight enough.
i re-positioned the neck in the up/down plane and re-tightened.
it fixed the problem!
altho it was fun making ghost-music by strumming a chord and then flexing the neck back and forth haha
dont know if this helps anyone, but it took me a while to figure it out :)
This is a commomly known phenomenon of the banjo, the neck can easily be bent, if so willed.
'Gourd?' 9 hrs
'Early Nitty gritty B,G.' 11 hrs