I chased a similar problem on a Norvel .061 setup I have.

First, you MUST get the low speed needle tuned. However, if there's too much play in the throttle linkage (a problem that plagues flexible steel throttle cables, especially if they must curve too much), on some, especially smaller engines, unstable idle can result. Get the mixtures right first, and if you still have troubles, think about this.

I fixed it by putting a light pen spring over the throttle cable, so that there was some tension on the cable all the time. Make it very light so as to not stress the servo, but strong enough to prevent throttle barrel floating, and it can help.

It's not a common fix to need, especially on engines over 0.10ci, and never should be needed on engines with straight throttle cable runs.

If you're using any threaded metal clevises in the throttle linkages, be sure to also use the jamb nut to prevent the threads from being sloppy.

J