I think you have some keys that stick. Have you tried a different keyboard?
Have you restarted since this started happening?
Go to your System Preferences, then International prefs, then the Input Menu tab. Click the box next to Keyboard Viewer. Now, click the flag menu that is now in your menubar. Click Show Keyboard Viewer. Now you will see an on-screen keyboard that will show each key as you press it, and (most important) will show you that each key releases when you release it. Test all your keys this way. You'll soon decide if the keyboard is good.
Serendipity is a lucky guess !