If you have surging...does it stop for a while with fresh spark-plugs? Mine did this with both the stock plugs and the Copper Cores and both seemed WAY cold for the driving situation around here. So..I put hotter plugs in and the engine runs without surging at all.
I know, I know....you'll destroy the engine with hotter plugs! Well...it hasn't happened yet in 60K miles so I'll just keep on destroying it...Thank You! Actually went with the +4 Bosch used in the Zetec as it fits and is hotter than the stock SVT plugs. They work real well for 'street' driving....Autobahn Burning maybe not so good. Have done multiple top-end runs and WOT's up a 4 mile mountain with no issues, but maybe wouldn't want to run extended WOT with them like you can out of the USA.