One problem not mentioned here, but I've fixed 3 starters that appeared to be having problems.

The common "hobby" 90 size and standard size starters have their power leads connected to the motor by two round terminal leads that are screwed to a plastic post.

In the starters that were having problems, the leads had over heated and some of the plastic melted and flowed between the tow round terminals and increased the resistance. Carefully remove the brushed end cap off the starter, disassemble the screwed together joint, clean off the plastic between them. Then solder them together and put the whole thing back together. Works like a champ, by lowering the resistance in the circuit.