I also assume you're doing it when the engine is up to temperature and not on cold start.
Take out the ISCV and thoroughly spray it with electrical contact cleaner. Push the plunger in and out a few times and clean it all out. Also, check the throttle body airways that the ISCV feeds air to and make sure they are clear.
That's the extent of my knowledge. Aside from that, others may be able to suggest things like throttle position sensor may need adjusting etc?
Maybe try checking your ECU for fault codes.
Hope this helps!