If the engine continued to run with the ISCV un-plugged, then I would suspect an air leak on the inlet system somewhere.

The RS2000 16v has a similar setup, and an air leak on the inlet system cause's the engine to run on after the ISCV is un-plugged, and idles high with it plugged in.

A quick test is to spray some WD40 around the inlet system with the engine running, any change in the engine note as you spray, highlights a potential leak spot. Don't forget the area around the fuel rail to inlet manifold seal, at each injector, as these round thick washers have a habit of leaking over time.