I had heard people say not to use platinums/iridiums on older vehicles, and I finally found out why! (As I use iridiums myself)

When you alter the material of the spark plug, you alter it's heat range. Iridiums and platinums are extremely cold plugs, meaning they will almost never pre-ignite the A/F mix, but it also means they never really come close to their operating range, resulting in them fouling very quickly and being "too easy" to jump as a result of the carbon track. If you are going to run them, about the only way to compensate is to gap them all to hell (Which makes the plug work harder, resulting in more heat) however that only works if the coil can actually jump the gap. You can see how that's problematic. As a result, you're probably better off just running the stock plug, as it's already optimised for your heat range.