might also want to try to locate another keyboard to test out in those circumstances, as well as unplugging and replugging the keyboard. I keep Keyboard Viewer accessible in the menubar for just such situations, so I can see if I have a stuck key or something. Has your keyboard suffered a liquid spill recently?
Keyboard Viewer can vary slightly in its location depending on your OS. You turn it on using system preferences, keyboard, international. It's one of the top language options, and when selected will cause the input menu widget to appear up in the toolbar near the clock. Access it from there when you want to see precisely what's going on with your keyboard. I've used it many times to diagnose stuck keys, often caused by liquid spills.
