Had a van once that did the same thing.
The coil had a crack in the top of it, that allowed some of the
oil to get out. The mounting bracket had a sharp edge to it, and so every time we would replace the coil (along with everything else trying to "nuke" the problem), it would crack the top of the coil again.
Finally an old guy at a no where gas station in Nebraska showed me what was wrong, and why.
Eric