What he ^^ says, it sounds like an air leak to me, check the famous T piece on the vaccum hoses which splits and thus causes an air leak, which would cause what you describe!

Yes if you coast with the car out of gear AND the clutch released the revs will raise this is perfectly normal and shows the VSS is working as this is where this is the input responsible for doing so! If you hold the clutch in and coast you will notice the revs won't rise!

ISCV is opening causing the high revving and shows it is working! If the revs are dropping to the point of stalling the ISCV will bring them up to prevent it from happening.

Martin