My car has been hard to start lately and I couldn't figure it out so I had it towed to a local shop, he had it in his shop all nice and warm and didn't have any trouble starting it so I picked it up, drove it home and it sat overnight where I went to go start it in the morning and it wouldn't start it just cranks over. Also, he said the spark was weak.

 

Weak spark, probably time for a new coil.

 

I had something similar and still having a problem, thought it was the battery until I noticed the new battery did same thing. It will start after the battery is fully charged, and then sometimes the battery loses 50% of its charge as measured with a volt meter. Its like something is killing my battery. Then the car wont start. It was a little different on mine though, cause the car doesnt turn over at all when the batterly loses its charge all of the sudden.