1996 Volkswagen Polo ignition key won`t turn

Hi there i have an intermittent problem with the ignition key not turning in the lock.Sometimes it can take 10 mins of fiddling about with the key to free up the lock.I have tried spraying wd 40 into the lock with no effect.Car has done 98000 miles,is this a common problem ,can it be fixed?anybody know a trick that might cure this problem.Problem is worse when the ambient temperature is higher.