Please guys... use cheap plugs. The NGK variety works best in these cars. They're like $6 a set, so you could replace these with more frequency than platinum or iridium or splitfires... whatever. If you want better performance with your spark plugs, index them. If you don't know what indexing spark plugs is, google it. Never install ANY plug without checking the gap. I think the tool for it costs $.50.

If that's the only thing that's non-stock or that has changed, try putting your old plugs back in. I know it seems silly, but you must eliminate variables.

I still think it's a vacuum leak. Engines don't rev on spark alone. There has to be too much air coming into the intake manifold to make it rev higher than idle.