The ISCV won't cause spluttering etc, I'd be looking at other things, plugs, fuel filter, CO ?
The ISCV can be extremely stuck sometimes, the trick I use is to insert a thin bladed flat screwdriver in the tiny gap between iscv body and the surrounding housing, and rock it one way, then insert the s/driver, rock the other way it nudges it out that way, as it stops the rocking motion of the valve, so long as the two screws and metal plate they hold are removed there is nothing else holding it, just the o ring on the end inside.
tabetha