Posting piccies would really help as it may show something yer missing.
Pull the starter and post piccies of yer contacts...that could tell us a lot. You could also take yer starter to AutoZone and they will test it for you. That could possible eliminate anything to do with the starter. If is not the starter, I would look at ALL the cables.
How good are the starter cables? How good are the battery cables? How good is that ground? Those would probably be the easiest to check/clean/tighten/replace.
