don't clean the air filter just replace it mate. As for the throttle body, not sure what it's like on a non GTi but you'll just need some Carb cleaner and a old rag, and just clean all the air intake if it's dead mucky upto and including the throttle body...but really you need to replace the plugs and give it a good service and then see if it makes a difference.. It could be a number of things if that doesn't solve it.