I don't think it's the ISCV. As you describe it, it's running fine when cold.

First of all I'd check for air leaks. Small ones might not cause any trouble when the engine is cold as there's enough fuel. But when the engine is warmed up it might stop running because of less fuel.
A broken MAF-sensor might also caus similar problems.

Does your car have a catalyst? In Germany all EFIs had one, (early) cars in the UK also came without. A blocked catalyst MIGHT cause such problems, but then it probably also won't run fine when cold.

There's a lot more reasons, but you'll need proper equipment to check. (fuel pressure, CO etc.)