X2
Sounds like it might be a long lead injector. It was Ford's fix for a problem known as "cackle". It was a problem caused by inadequate fuel getting to the #8 injector. Their brillant fix was to put in a modified injector instead of fixing the problem with the fuel system.
If you are experiencing a "cackle" problem, shimming the FPR may help out some. Making sure you aren't getting any air in the system will also help. The "Hutch" mod takes care of those typical problem point where air gets sucked into the fuel.