Please Help Me Guys

Hello Everyone
I Have An iPhone 3GS (iOs 5.1.1 Baseband 06.15.00) It Shuts Down Randomly And Always Shows The Apple Logo When I Try To Turn It On, But It Does Turn On When I Put It Charging, And When It Is Charging, No Matter How Long I Use It, It Doesn't Shut Down)
My Second Problem Is Brightness, The iPhone's Brightness Stops Sometimes, The Cell Phone Stays On And I Use It Normally But I Barely See Anything. Sometimes It Stops And Sometimes It's Back...
This Is Griving Me Crazy, Please Help Me Guys And Thank You

And By The Way, When The Phone Is Standby, It Never Shuts Down But When I Start Surfing Net Or Playing Games It Shuts Down