Hey everyone! I know it's been over a year, but I believe if figured this one out. I thought I'd post my theory, then update when I get the parts in and the gearbox back together.
Basically, the problem with down shifting seemed to go away, but it started creeping back. I was riding one day and the issue got worse. Then the selector spring broke, so I had to get the bike home in 2nd gear (thank you to everyone that posted on the forum about the springs breaking; it made my problem easy to diagnose).
I got the new selector spring in and was putting the gearbox selector back together when I noticed ANOTHER broken spring, the Lever Spring. So, I believe that my troubles down shifting were because one of the long arms of this spring had snapped off. I also think this led to more stress than normal on the shifter spring, which caused it's failure, or it could just be a coincidence. Either way, I'm ordering a new lever spring from Harper Moto. Like I said, I'll update when I get her back together. Hopefully, these were the only gremlins in the gearbox!
On another note, anyone have any advice on installing the circlips that hold the selector gears in place? I always seem to bend them out of shape, rendering them useless.