Battery cables. Ground.
![Werd :werd:](https://board.marlincrawler.com/Smileys/marlin/werd.gif)
Make sure your battery cables are not corroded, make sure all connections are strong and clean.
I have the same issue in my rig and it's been going since I got it. The clean battery terminals/cables help, but the only thing that'll fix it I think is a new starter. Did you try banging on the starter with a hammer?
![Distrubed :disturbed:](https://board.marlincrawler.com/Smileys/marlin/disturbed.gif)
That works for me (sometimes).