I've noticed Family Guy do this quite a lot too. In an episode I saw recently (season 4 episode 4). Meg is turned down by a boy at school and ends up having a makeover at the mall (plot 1). Meanwhile, Peter, Joe, Cleveland and Quagmire try to stop the Clam going out of business and discover Karaoke as a solution (plot 2). So they decide to start a band but their first gig goes badly and so Peters family step in and put on a good performance (plot 3). Peter's family then go on tour and Meg, now the attractive lead singer of the band, lets success go to her head (plot 4). Meanwhile Brian tries to suppress his racism (subplot to plot 4).