Sounds like you either have a bad connection somewhere (if you are using a panel) or the brushes are going bad. Check all the connections on your power panel make sure none of them have been overheated. You will notice a slight scorching of the wire shielding next to the solder or worse the entire shield is a bit malformed from heat.If this is the case replace the wire at the bad connection then try your starter.Also a great idea to check out the switch contacts on the starter.

Another thing to check, take the caps of of both ends of the starter, if it has drawn moisture it could just need a simple cleaning. Use a fine grit sand paper to shine the armature.
the high torque starter uses gear reduction to up the power at the output shaft, could just need greased up. If the grease has dried up some it will be a graphite loaded glue.

You can easily replace the brushes in these starters go to your local auto parts store and buy a set of alternator brushes for an 85 ford pickup with a 302 they are like 7 dollars. File them to easily fit within the guides solder the wires in place. The brushes are spring loaded to constantly touch the armature of the starter.So anytime your spinning it, it is wearing graphite with metal. You may get lucky and take your starter to a motorcycle shop and they can order the brushes to fit.