BB Curve :: Trackball Won't Scroll But Isn't Physically Stuck

I've been having a bizzare issue with my Blackberry 8900. Basically, the trackball will just stop scrolling and I can't move anywhere. But the trackball itself doesn't feel physically stuck, it moves the same as always, but it won't move on the screen. It does, however, let me select still using the trackball. The menus aren't frozen, it's just a scrolling issue.

I've tried re-installing the software and have the most recent update available. I've tried restoring it to factory settings, but it's no good. The problem is intermittent - about 90% of the time it's frozen. The only thing I've discovered to remedy it is a battery and sim pull - this worked for a while, and I was able to use the phone as normal, but it stop scrolling and need a sim pull approx every 24 hours. Now that doesn't even work. I gave up trying to use it and put my sim in my old Blackberry, but turned my 8900 on anyway (simless) and it worked! I thought it was a fluke at first, but I've been trying it all week - keeping my phone on, simless, and noting that it works all the time - no problems at all. Occasionally I'll try putting my sim back in my 8900 and sure enough it breaks again. So I started to suspect there was an issue with the sim. I called up Orange and got them to send me out a new one, just in case that was the issue. They've done that, it's all registered, but still I'm having the same problem - it won't let me scroll when there's an active sim inside, but as soon as I take it out it's like there's no problem at all.