1999 Ford Ranger starter engaging

I have a 1999 ford ranger xlt 6 cyl 3.0 I have gone through 4 starters in 3 weeks. The starter engages by itself while I am driving. There is a poping sound followed by grinding from the starter. After loosing 3 I noticed when it happened I would turn off the key, park, and turn on and off the key switch. I noticed that when moved the key to crank the engine. It would start up winding without turning the key all the way After several attempts it would start without a problem. But for how long I did not know. Sometime it would last several days, sometimes five minutes. I notice when the poping sound was there dash lights would flicker on and off. I have replaced the starter switch under the dash, with no improvement. Could this be something to do with the ignition lock and key since I have the anti-theft? Going haywire and engaging the starter?