Very strong probability that the rubber vacuum hose which connects to the air intake flap servo motor has deteriorated where it meets the servo resulting in loss of vacuum. If you look down the air intake you will see the flap - the servo is at one end of that.
Easy fix: just pull it off, snip the end so it's neat again and reconnect to the servo. The servo is located under the bonnet at the end of the air intake flap (the end nearest the wing).