How often do you clean out your machine? I've a little rule: every change of bobbin means a quick fettle around with the cleaning brush, and every now and then I take the vacuum cleaner nozzle and give the whole machine a 'deep' clean. Remember, a machine that's full of dust and fluff isn't likely to run well and could have tension issue.
A clean machine is a happy machine and a happy machine means a contented sewer!
Hazel & Terry