Here is something I really can't understand at all. Apparently modern cars are so tough that if a professional mechanic can't get a diagnostic handed to them from a machine, they just throw up their hands and say "duh, never seen that before" (which is same thing I would usually say). Ok, fine - it is what it is; but why can't they at least round up the usual suspects? Yes, we know that there are lots of possibilities for the P0300, but some of them are fairly well-known, and should be easy and simple for a Stealership to try (sorry about the profanity Vanessa). It's the same thing that we do, only they don't need to buy the parts. Why does an Aveo dealer not have one set of good plugs, wires, coil, cam sensor (and maybe some other easy ones I'm not thinking of) on hand, ready to try as temps. How long is it going to take a pro to swap those things out? First ask the customer if they want to pay for a part-sway try (labor only). Then, if something works, take out the temp and install a new one off the shelf - case solved. If nothing works, bill the customer for the time to install/remove the parts.
Ok, that's the end of my useless rant. Personally, I would first try a new set of OEM plugs (regardless of how new the existing ones are). Then, maybe go to a boneyard and pick up a basketfull of parts like the ones above. Might as well try hanging parts if the shop can't figure it out