I know the frustration, buddy!
I've taken apart my Warthog probably a dozen times, and spent dozens of hours staring and working on my stiction issue. I too got to where my roll axis was buttery smooth, but it would still hang up on the pitch axis. I eventually got it to an acceptable state where the stiction is almost non existent all the time. Although it seems I sometimes need to just run my joystick through the motions to spread the grease before I hop into the pit.
That being said, I unfortunately could not say what in particular was the problem that was causing the stiction issue on the pitch axis as I would end up trying multiple things when I took it apart in desperate attempts to just solve the problem.
There was two different things I did that I know made a noticeable difference after the sanding and re-greasing was done though:
1. If you haven't tried it yet, I would suggest removing the large centering spring that sits on top of the pressure plate, and use the smaller springs on the bottom in its place and just using it that way for a little while.
Sure the centering isn't as great, but I have found it plenty good enough an never had to use dead-zones or anything afterwards. In my case, the spring was simply putting too much downward pressure on the gimbal and creating unnecessary friction. This by itself made a huge difference.
2. I found that the holes in the white plastic piece that is screwed onto the top of the pressure plate weren't lined up perfectly with the holes in the pressure plate itself so I sanded the holes open a little bit on the white plastic piece. Since it wasn't perfectly aligned, it was rubbing on the poles constantly. This wasn't as big of a problem but it did help a little bit.
Hope this helps you out a bit. If your like me its all you can think about when flying an it takes away a lot of enjoyment. Thankfully I've been able to alleviate the problem over time to where its almost non-existent although I'm sure I'll take it apart again at some point to try and make it perfect.