I too was experiencing rough idle on my 4.5, while running plats, although I didn't notice the roughness until after the car was down about 3 weeks for repair. After reading multiple responses here regarding platinums, I picked up a set of Bosch copper cores. The idle has smoothed considerably and power is increased. I also noticed increased mileage on the last tank, which fed my normal drive as well as two full throttle runs past the century mark to blow out the crud (exit to exit on the interstate).
Prior to changing the plugs, an interstate run would clean the system for about a week. After a week, I would notice black smoke under hard acceleration. Always thought it was just build up from stop and go runs to and from the office. Besides causing a rough idle, the plats apparently caused the engine to run a bit rich, as evidenced by black deposits around the electrodes. It's been a little over a week since the last run and I had no black smoke today under acceleration.
The book I have states my gap should be between .028 and .032, I set them all to .032.
73 280 SEL 4.5