My trackball would do everything but scroll down, before I cleaned it.
I removed the trackball unit from my 8310, but I left the ball and scroll wheels intact. I set the unit in a small bowl of rubbing alcohol and let it soak for about 10 minutes. I pulled it out of the alcohol after 5 minutes and rolled the trackball in each direction. Rolling the ball when it's wet with alcohol helps the cleaning process. At that point, scrolling down still felt a bit stiff, so I swished it around in the bowl and soaked it for 5 more minutes. Then, I removed it again from the alcohol, rolled the ball around and set it out to dry.
The scroll wheel for scrolling down looked somewhat worn, compared to the other scroll wheels. I'm sure this is because I do more scrolling down than scrolling sideways or up, especially when I'm reading long emails. The scroll down wheel was lighter in color and just a bit looser than the others. When I put the trackball unit back in, I rotated it a half turn so that the worn scroll down wheel is now the scroll up wheel. This should help with the issue of uneven wear, and it didn't interfere with the up and down functioning of the trackball.
While the trackball soaked, I took apart most of the rest of the 8310 so I could clean the keyboard. Check youtube for videos on taking apart the 8310.
I soaked the keyboard in alcohol for 60 seconds, then blasted it with compressed air, front and back. This got rid of all the grime between the keys. Now my Curve works better than ever! The keyboard is highly responsive, and the trackball scrolls like a dream!
Be careful during disassembly and reassembly. There are little plastic clips that can bend and break.