90 Ranger, starter stays on even with key off

This has happened a couple of times now. Turn the key to start the truck and the starter keeps running even after the key is off. Have to disconnect battery cable to stop it. Replaced starter and solinoid switch 2 weeks ago. Happened again last night. This time I had to short the terminals on the switch with a screwdriver to get it to shut off. In the last 2 years this has happened about 5 times. I've replaced the solinoid switch 3 times. Why does this happen?:eek7:

Some of our posters on the Escort forum had the same problem. It was fixed by putting in a new ignition switch.