Courtesy of Eric (not his blog, however — from a jokes mailing list he maintains):

A man went to the doctor with a strange complaint.

“Well, it’s like this Doc, when I drive to work in the morning through the country lanes I start to sing ‘The Green Green Grass of Home’.

“If I see a cat I bust out with ‘What’s new Pussycat?’ It’s so embarrassing, even when I’m asleep and dreaming.

“Last night I work up singing “Delilah’, and my wife was not amused!”

“Hmm. it would appear that you have the early symptoms of Tom Jones syndrome.”

“Well I’ve never heard of that, is it common?” asked the man.

“It’s not unusual,” replied the doctor.