MS Intellipoint scroll wheel (mostly) stopped working

I use a Microsoft Trackball Explorer and MS's IntelliPoint software v. 5.2 within Win XP Pro XP2. I also use Microsoft's Wireless Comfort keyboard. Both are connected to USB ports.

The problem: Recently, the scroll wheel on the Trackball stopped working most of the time. I say "mostly" because I can occasionally see a few lines jump when I turn the wheel, but most of the time, nothing happens when scrolling.

This appears to be related to Intellipoint. I uninstalled Intellipoint and rebooted with the standard mouse drivers, and the scroll wheel worked fine. So it's not the scroll wheel itself. But I do want to use Intellipoint to get the extra functionality for the extra Trackball buttons.

Here's what I've tried so far, without success:

--Uninstall the Trackball in Device Manager and reboot, letting Windows reinstall.

--Uninstalling Intellipoint, rebooting, and reinstalling Intellipoint.

--Switching the USB  ports for the Trackball and keyboard.

--Making sure that the wireless mouse that came with the keyboard is off.

--Making sure that the Synaptics touchpad on my Thinkpad is disabled. (This is a Thinkpad T40 attached to a docking station.)

Any other suggestions?

Thanks.