The usual fix is to crank down on the bolt that goes through the center of the right friction shift lever, combined with considering learning to pedal smoother. The friction at that point holds the bike in gear. It's not designed for loctite. The trade-off is concern about stripping the bolt or the hole in the frame. At least the ones with the little handles on the screw are intended to be tightened with hand strength without the leverage of a wrench. I don't know how your loctite has affected that. Some of the early Campy ones (Valentino or Gran Sport) have heads for flat screwdriver, so you have to be more cautious with them.

The number of rear sprockets should not matter, to my mind. You need to stop the cable from slipping as the bike flexes. Another possible issue is making sure the cable path is clean and cable friction is minimized. Under or near the BB shell I would be concerned about this.