Engine spins...But

My cars started having a bit of a problem cranking this last week or so. Basically, you push the key into the start position, and the engine will turn over as long as you hold it. Then, as soon as you release the key back into the run position, the engine catches and starts up. Its a minor annoyance, but still.
I replaced the lock cyl. yesterday for an unrelated problem, and was hoping that'd maybe fix it, but no luck.
Am I looking at my ign. switch itself crapping out? Or might this be something farther down the line?
BTW, we're talking about a pretty much stock 93 GT, 5 speed.

Thanks,
J