I agree with the starter. Had the same thing on VW diesels. Suggest new starter with new armature as well as bearings and brushes. I know people who put new bearings and brushes in and cleaned up the starter only to find minor improvement that diminished quickly. If you have already re-built the engine-cost of a new starter is worth it. One other remote possibility first. I had a VW diesel that intermittently exhibited the same problem you have. Sometimes it cranked properly and sometimes not. New starter did not cure problem. The positive cable from the battery to the starter was corroded inside the cable so you could not see the corrosion until the sheathing was cut open. The high resistance of the wire caused the problem. Suggest you try new positive cable- don't know if your car uses a ground cable also - if so change it and clean the point where it bolts to chassis. Then apply a film of vaseline to the connection to retard further corrsion. Hope this helps