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

 

Could be the air flow sensor. Or more likely the speed sensor, they're the cause of many running problems, as they send info to the computer, and they tend to get fouled by the elements (and are a bugger to remove/refit, have to do it from under the car).