Starter cranks without key in ignition

[Your41Plague12]

Ok guys, here's one for you. Every once in a while, although it's happening more frequently now, I'll turn my key to start my truck and with the engine running or not, my starter keeps turning. I can turn the key and shut the engine off but the starter keeps turning. It does this even with the key out of the ignition! Is this just a bad ignition switch? Or is it something starter related? It works fine 95% of the time, but every once in a while I'll have to go out, pop my hood, and disconnect the battery. This is really frustrating. The engine is a crate 350, the tranny is a TH400, and the starter is GM with a recently replaced solenoid. I hope this has happened to someone and it's something simple. Thanks in advance!

Steve