I have a 1994 460 F250 and it has the same thing rough idle I have changed the spark plugs wires and distributor..It started when I used the back tank which when dropped and cleaned out had all sorts of crap due to the air vent thing on top not been on...I was able to put on with the tank off (hard to get in there) it made my engine miss and have no power to go over 45-50 AND this was towing my 22 ft fifth wheel...I figured after the cleaning and running a bunch of Lucas cleaner in the gas it would get better and it did for a bit..NOT GREAT but better from the front tank...Once after cleaning the back tank etc etc I put some gas tried the rear tank again and it is a bit better but not like before and the miss seems to be at the 1000-1400 RPM, i checked the spark plugs, made sure they were not crossed etc but now it is running worse so I am thinking something in the ignition or fuel but they are both new and new fuel filter too..ANY ideas are very welcome at this point as I am going to need to tow my 11000 big fifth wheel to the Grand Canyon in a few weeks and want to do over 15 MPH