Could it be a hardware problem?
Perhaps incompatible hardware? I heard Nvidia cards may cause some exotic bugs, and if that happens you should try switching to software rendering.

If it's not likely a hardware problem, then how about trying to start a new game and see what happens? Maybe your save file got corrupted or something.

Also as a side note, since you're still not far into the game and in case nothing else works, I'd suggest backing up your save files and re-installing the game with the recommended mods.
The widescreen+ui mods are beautiful. ^^

Have you got a gamepad or joystick connected ?

Or a mouse that has seperate drivers (eg Logitech) ?

I experience this same issue on a brand new install with a new game. This has happened to me with three fresh installs. The issue occurs after approximately 8 seconds. Even after using the map to place the screen to display the character again, ~8 seconds later, the screen starts scrolling in a fixed direction.

I'm wondering if it is a bug with the infinity engine because I also experience an issue with Baldur's gate after 8 seconds where the dialog box opens up and can no longer be closed. I am currently experiencing this issue with Baldur's gate and I wonder if I stop experiencing that issue, if I will stop experiencing the mouse scroll issue in Planescape.
for cross referencing:

http://www.gog.com/forum/planescape_torment/strange_camera_panning_behavior/page1