I'm not exactly sure what "stumbling" mean... at least for Idle (up and down?)!

But if you mean also oscillation!(800rpm-1300rpm) it's a mixture of 1 to/and
3 thing, most of the time... off timming (or of course a bad or worned
distributor) / loose throttle body / offset AFM.

Most of the time, i fix that with a 10mm wrench (timming) and a flat screw
driver (to lower idle)... the timming as too much retard, so someone one day
unscrewed the idle screw to compensate the low idle. Put more advance and
lower the idle... it sound to simple, try it before you say anything.

My 2 Canadian cents
Ben