@Matt , sounds like the same issue I’ve had (and others) in the past. The Z is very sensitive to binding. So spinning the stepper at near max speeds when there is anything to resist it will be problematic. I’ve also greased mine and it helps abundantly.
I do run mine at 8.5mm/s, but the truth is, you don’t really need to have the Z go up and down at high speeds to have a fast job. Repeatedly consistent operation of the CNC is preferable, as I’m sure you’ll agree.
So why not just set it to 30% less and see if it remains reliable. As your threaded rod wears, and becomes easier to turn, you may be able to bump it up. So maybe creating a gcode file that “burns in” your threated rod by running it up and down a bunch of times throughout it’s range might help. Running a die and tap wouldn’t hurt, but the burn in and grease should do the trick.
Let us know how it turns out, no pun intended 🙂