Could it be starter?

[cgster]

I've been slowly working on my 72 C10 for a couple years, I try to crank it every 1 or 2 weeks and let it run for a while or drive it around the yard. The wiring on the truck is a mess(someone installed alarm and sound system years ago thats been taken out), but really hasn't given me any problems. For the past few months, when I go to try and start it, I turn the key and it takes a second before it even tries to start turning over the engine. Sometimes nothing happens at all when I turn the key, try a few more times and then the engine will finally start turning over. Then when the engine starts turning over it just never ends up cranking. This comes and goes. One day I can go try to crank it, if fires right up. Next time I can try for a few minutes and it never cranks. Battery isn't the issue as I've tried 2 new batteries. I don't think its an engine issue, the engine has been rebuilt with less that 150 miles on it. Could this be the starter giving me this problem or more of an electrical issue?