2000 Chevy S-10 Engine Cranks Very Slowly

I own a 2000 Chevrolet S10 with just over 100K miles on it. Since I've owned this truck for 2 months it has started very slowly. The dealer told me the battery needed to be charged, but after that was done I still have the same problem. I've replaced the starter twice, the one installed is brand new. The batter is an Interstate Battery. I have had the battery checked by Interstate and it puts out plenty of cold cranking amps, and all the cells are in working order. My problem is that when I try to start the truck, it cranks very slowly like the starter was bad. Sometimes after 3 quick starts, the starter will spin but the engine won't turn over at all. Most of the time, it sounds like it just gets enough power to turn over and then it starts. Also the battery output on the dash is showing right between 9 & 14. I'm not sure if the battery output has anything to do with it, or if it's right where it should be. Any ideas?