Car batter/battery/Alternator problem

Hi,
Recently I have faced a new issue with my car.
Sometimes and not always when I try to start my car, it does not.
The lights on the dash board appears initially but once the car does not
start they go off.

I then go and use my keys to hit the negative terminal of the battery. Once I do it for a couple of times(hit the terminal with key for about 10-15 times). All lights come up on and then I can start the car.

-- The terminal is not corroded.
-- The terminal does not look loose.
-- The battery has been checked and is GOOD

I am worried what else can be the problem? Please help..

I would replace the ground cable and terminal, then the battery if the cable doesn't fix the problem. Intermittent connections are tough to determine.