I have a 115ps 1.8 Si (which i've just converted to 130ps inlet manifold)

Currently, and prior to the 130ps conversion - the engine has developed a misfire below 2000rpm. It helps the engine to cut out if you leave it idling by itself and dont keep it at 2000rpm

I have fitted new plugs and leads, but don't know where to go next?

Any ideas would be greatly appreciated!