Ford Repair: Starter, starter solenoid, ford e150

Question
1990 Ford E150, 5.0, Auto Trans:  On occassion, the starter keeps turning over, even after the engine has started, and will not stop, even if the ignition switch is turned off.  It stops when it's ready (I managed to disconnect a battery cable once and that stopped it), and afterwards, will start as normal until the next incident, usually months later.  I've replaced the starter maybe annually over the past 3-4 years,and because it's my "hunting vehicle", I've accepted that in-lieu of an open ended electrical evaluation that could potentially cost more than the worth of the van. Any ideas as to what is the cause?  Thanks.  

Answer
Hello and thanks for the question...I'll try to help.
 There are several things that could be causing this to happen.
1. A stuck fender mounter starter solenoid.
2. A engaged ignition key.
3. A faulty starter.
4. Faulty starter/battery cables.
Hope this helps,
Erik