1998 Buick Century would not start after driving it around

I had made a few short trips yesterday and coming out of a store it would not start. No clicking or any noises. Lights on dash came on but totally dead. I had it jumped and it started right away. This hapened to me in October of 2009, but has not happened since.I did not get it looked out then. So March 2010; 6 months later it did it again. It has a Interstate battery and was purchased about 8 years ago. I did have the Auto Zone attach it's battery machine and he said the battery seemed strong in Oct. 09. I am due for a oil change, which I usually get every 3,000 but I'm 1000 over. I had everything looked at in Summer 09. New hose put in two weeks ago because coolant was spurting out due to the little hose up by the power steering container. Yes that fixed the leak. What do you think? Thank You, AnnMarie