deadpair,
I feel for you, as I have battled with the same thing on & off for years. Everything posted so far are things for you to look at and consider, and many more suggestions to you will come on this thread. I'll add a couple..
1 - Don't use a rubber grommet on the shot bottle side (right). It can catch, but only sometimes.
2 - Snug both bottles, but then back them off just a bit.
3 - On the upstroke, be sure to push that handle back. (Yes, I now the unit giving you fits is on an Automate, but 9000Gs without them will do the same thing).
4 - I use the tall bottles on both sides, with a bottle support. Swap them out every now and then because the graphite on the inside of the shot bottle seems to provide a slippery lubed surface to allow the powder flakes to fill or flow.
5 - Wash the bottles, particularly the powder bottle, with some dish soap. Rinse in cold water, and naturally allow them to dry totally.
6 - Watch the screw in the bar go all the way to the left, and then all the way back to the right. This forces you to spot any drag in the bar.
7 - Promo will be even worse than Red Dot just due to the inconsistency regarding Promo powder.
8 - This is the last one. Go up one bushing size, say a #31 vs. a #30. That would increase your powder drop a half a grain or so, which puts you a little further up in the "range". Then, when your powder drop is light by 3/4 of a grain as you stated above, (and yes, I agree with you on that), the shell still has enough velocity over a chrono to do what you want/need it to do.
Good hunting. I will watch this thread with interest.
Jon Reitz