I just changed the plugs. Drove it around and it seems to be chugging a bit. I pulled out each new plug and it looks like the back one (the hardest one to replace) isn't firing. All the others have brown tips (like they've been firing), but this one, which is clean. I tried reseating the plug and the coil (several times). Still no luck. The car drives, but as soon as I accelerate hard, the CEL starts blinking. unplugged and replugged the coil... nothing. Any ideas?