<VV> starter woes

[Douglas Barneck]

Two of my cars have the same starter symptom.  While starting to crank, if the engine starts quickly, there are no problems.  If the starter has to crank for more than 10 seconds or so, the starter drive will engage and then it sounds like it starts to freewheel and not turn the engine.  I can hear the starter still turning.  I replaced one of the starters with a spare that I had and worked normally for a while but now it apparently does the same thing.  Any ideas on the cause?  I think it is odd that two of my three daily drivers do the same thing.
Thanks
Doug Barneck