I get TV theme songs stuck in my head all of the time. It's a problem. Oddly enough, the longer it's been since I've seen a show, the more likely I am to realize that I'm singing the theme song. It happens most often with cartoons that I used to watch as a kid. I may not have seen that show in years, but I'll just start singing the theme song out of nowhere. From there, other cartoon theme songs start popping into my head, and it continues for the rest of the day. I'd say this happens roughly once a week or so at the very least.
The worst is the rare occasion when a theme song gets stuck in my head, and for some reason I decide that I'm going to look up the theme song on Youtube, even though I know what will happen. Then I spend at least an hour going through theme songs of old 90's cartoons. Sometimes this eventually turns into watching theme songs from live action shows as well or even newer shows. However, that may be worse than the other option, which is it turning into me watching actual whole episodes of these cartoons if they're on Youtube. I can waste an insane amount of time if I get sentimental.
Netflix got rid of all of the old Nickelodeon cartoons a few months back, and I was insanely disappointed. It was probably more of a positive than a negative though, since I don't really need Netflix making these sentimental moments even more time wasting.
One thing that I do get from these type of moments though: I need to stock up on DVDs of all of these old shows. Both just for myself, but also for my future children. You better believe that I'm forcing them to watch every single show I liked as a kid.