Clean your battery connections. I had the same thing happen, and because at the time I had all sorts of custom wired goodies in my car, I thought it might be one of those wires semi-grounding. Turns out, all I needed was a good polish to the battery terminals and some new clamps. I'd do that before messing with replacing the battery or wires, it'll save you a lot of money if thats all it is.

 

i had the same problem when my CAI was hitting the battery terminal. you had to floor it to get it to start.