Is the thread new? I just learned this past week that thread does get old and rots. If you give it a slight tug on the thread if it breaks its rotten. I was told that sometimes NEW spools are rotten sometimes too but not very often.
You might want to empty out the bobbin (I know its wasting) but refill your bobbin. Change your needle, clean out the bobbin area really good too. Could you be using the wrong needle for the fabric your using?
I'm just rattling here so I am sure that someone else will come along and tell you what to do thats more knowledgeable then I am.