When this occurs I normally think 'hey, I'll oil it, see what happens' as I know I don't oil my machine often enough. Normally this sorts it out.
(The Singer website says that if you sew every day, oil once a week, if you sew once a week, oil every month. However I have a modern machine and most of them nowadays are 'self oiling' and I'm normally too busy sewing to remember I need to do it!)
I oil mine every two to three months. Quite a few of my clothing alterations I do by hand, especially if it's just a hem or a button that needs sewing on. Sometimes I think it needs oiling more often but considering how much and for how long I use my machine I think once every other month is just fine.