Misfiring, revs low, losing power

Started by just cutting out and staring again when pulling away/while driving. Note will start fine, but as soon as revs down, including when you've just revved it, the revs just keep going down until sometimes the car cuts out. Instead of just starting running revving, revs going down and levelling out its like it's pulsing, trying to pick back up.
When you try to drive it, when you push down the throttle it takes a second or two to realise, then starts trying to pick up speed, but misfires and judders.
While sat running it misfires and judders, when revved it pulses as the revs go down until sometimes cutting out.
It has had: new fuel filter, new plugs, new leads, new crank sensor, throttle body been taken off and cleaned, air filter taken apart looks clean and fine. Nothing seems make a difference. It also sounds like it's making an air sucking noise somewhere under the bonnet.
Needs help asap :'(