head scratcher!!!

I have a fresh 440 that I just built. It started and ran beautifully I let it sit for a couple days and it acted as if the battery was dead. What I mean is that it turned over but really slow and then nothing. I charged the battery thinking that would do the trick, but it didn't. I went out and bought a new starter relay, replaced my old starter wires, had the battery checked out, and still the same problem. I went out bought a new starter and put it in. It started right up with no issues. I ran it for about ten minutes and shut it off. I then tried to start it again and had the same problem all over again. Please help me think this through as I am at my wits end!!!!