If your machine's manual shows you where to oil and lubricate your machine, then it is relatively easy to do, and you don't really need to take it in for service. Be sure and use a good sewing machine oil (Singer is good, or there may have been some included when you purchased the machine). If it calls for a lubricant, do not use oil. Singer lubricant comes in a tube. Clean out all the lint and threads that you see, especially around the bobbin area, which you said you do. Many times, when thread starts breaking, it is because the machine was threaded incorrectly. You didn't say whether you had changed the thread before it started breaking, but I suggest that you try again with your original thread and just rethread the machine.