2000 metro starting problem

I have a 2000 chevy metro and am having problems with it not starting on an intermittent basis. One day it will start fine and another it will turn over but not start for 4 or 5 times. Sometimes when it does this it will turn over normally others it will be slow. I had the battery and charge system checked and they are good but the battery had a low charge. I have charged the battery several times and changed the spark plugs which helped somewhat but still get the problem. Any ideas?
Thanks
Tim

When was the last time the fuel filter was changed???

I just bought the car 2 months ago, I haven't changed it.
Thanks