Hello i brought my focus about a month ago

Intemittently the car will when changing gear hold the revs Where They were eg 2000 for a second then drop
slowly, also when comming to a stop it will be ideling at 800 as normal then rais to 1100 as i stop for a moment
with an odd popping from air filter box?

And other times will be seemingly fine i have no fault
codes and i sprayed easy start about the inlet/vaccume
and cleand the idle control

Hope for some advice guys thank you for your time