If you are not having issues now it's got nothing ti do with the new plugs being platinum. You changed from Champion to Motorcraft is where your improvement came from. Every Ford I (and my dad) have ever been around ran best with motorcraft/autolite coppers, with the sole exception of the Probe I had it actually called for and ran best on NGK plugs. Just like to point out that includes a '68 F-100, 84 Ranger, 90 BII, '92 (I think) Tempo, '94 F-150, '99 Ranger, 02 Ranger (two of them) and an '02 Taurus. There have also been many older Fords my dad had before I was born including a Pinto, a few falcons, and a mustang. Personally, I've never put Champion plugs in an auto engine. Was told by my dad (there he is again noticing a trend?) that he never had anything but problem with Champion's auto plugs. He'll use them in lawn equipment when he can't get an NGK, and they make great aircraft plugs, but their automotive division sucks.
Back when I was a Tech at Dodge Champion was the best plug for those cars.. A Dodge ran like crap on any other plug.... It's crazy.