C K is right :) if you know how (its a simple enough job), you do need to clean around the edge of the screen. the dirt/grit may not be visible easily, but it pushes the edge of the touch sensor down a little, and makes it mis-read the position.
Yours sounds like its got a few grains of dirt on the left side, and its possible to use cleaning spray (foam cleaner usually, nothing too liquidy & NOTHING abrasive) on a SOFT toothbrush and brush down the left side. Take the battery off first, and hold the power button too to remove all power from the unit forst!! Just a basic engineering safety measure - better safe than sorry :)
After cleaning, run a soft paper tissue along the edge to soak up any excess wetness left, but really you dont want ANY wetness to be left.
Or take the base off, and take the insides out (disassembly guides are all over the internet), then gently clean both the screen edge, and also the casing plastic. Its sticky around the edge thats why it grips dirt and fails easily.
On mine, all characters are out further to the left (e.g. P gives P but G gives A etc. This is because the complete touch screen has been pressed down out of position :(
You need a TRI-WING screwdriver to take the base off the DS. All other screws including hte battery cover, are standard cross head type.
Good luck.
filesponge, December 2008