MY ford f250 1988 is trying to turn on by itself?

MY ford f250 1988 is trying to turn on by itself. I have replaced the starter and the solenoid 2 times and it worked for awhile but now its doing it again. I'm pretty sure its something causing the solenoid or starter to go bad but i can't figure out what. I would sure appreciate any help you can give.

That has to be a bad or worn out ignition switch if the switch is aligned properly. Those switches are pretty cheap.

A $25 common part - http://images.apwcontent.com/is/image/Autos/sius130_1?%24APW_imgProd%24