http://www.youtube.com/watch?v=4xJ6cVO_g9E&feature=related
This is an "Italian tune up". The old Ferrari's would foul out there plugs at low RPM's. So if you drive slow they will run rough, and you need to run them hard to blow all the crap out. The old story goes that the owners would drop them off at the mechanic's complaining about a poor idle, and the mechanic would take it up in the mountains to blow it out.
Diesels respond well to them as well, on modern cars they are just for fun.