Hi Roy,
You could have a dead spot on your touch screen.
While doing something other than typing, try to determine if your touch screen is registering touch in that same spot.
If not, look up the 3GS repair guide on iFixit guides. The replacement guide for the digitizer has a detailed step-by-step guide as well as links for the needed parts and tools.
There is a small chance that the touch screen connector on the board is broken or displaced from a drop. Using the same guide, and before ordering a new digitizer, try disconnecting then reseating the screen assembly and/or examine the connectors using a magnifying glass. If nothing is broken and reseating doesn't fix it, you can then attempt to replace the digitizer.