Mine did this and it turned out to just be a bad coil connection. The coil itself was fine, just needed to be reseated. The Variation you have with it running then restarting and running fine sounds about right too. Heat causes expansion and this coudl cause the coil to become seated better and run better, but means there is not enough connection to maintain a solid connection. I had a misfire on Cylinder 8 went down to BMW picked up a new coil, then thought, I'll just push this on better and see what happenes before I actually replace it. Syre enough. She fired up smoothly and ran great.
Hope this helps and this is just some bad connections for you, rather than what gebhardt is describing.