1994 Saturn SW2 Starter will not disingage after releasing

This one should stump you. After starting engine, the starter shaft stays engaged after engine begins running (you can hear it doesn't release by the noise the starter makes). After turning ignition key off to kill engine, starter continues to turn engine over until you disconnect negative battery cable. After reconnecting battery to try again, sometimes the starter disengages ok on the first try, but will not on the second try. This problem started with original starter, continued with 2nd and 3rd replacement starter. Replaced ignition switch, no help. Tested purple wire which engages solenoid by turning start switch several times, 12 volts drops to zero each time I release switch, so switch works fine and circuit seems ok. HELP!