I wouldn't be surprised if you've lost cyl #6, as in its not firing. That spark plug is exposed to a lot of heat, and is difficult to get out. If you've taken it to a shop for a tuneup in the past, there is a possibility that they didn't even change that plug. (I know of a few shops that do that) Do a full tuneup, change all the plugs, and check back. If that plug has been in there for a long time, its prolly been cooked.
