Many times the starter turning over slowly is directly related to poor grounds. Make sure ALL your grounds are touching BARE metal. Paint, undercoat, corrosion, dirt and dried grease don’t flow current very well. Scrape and wire brush them all. The fact that you have had your starter apart is suspect also. When I first read your post I was having a hard time understanding why a truck engine would have 2 coils then realized your were talking about starter field coils not ignition coils. Good luck.
