Have you tried jumping out the starter relay? The extra boost could make the relay close a little tighter and engage the starter but not engage under normal conditions. If you tap on the starter with a wrench or hammer while the key is turned, will it crank? If it does, the brushes are sticking. If the relay and brushes are OK and it just has a very slow or barely there crankover then it sounds like the starter motor may have a bad armature winding. In that case, replace the starter.
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