There can be lint stuck between the tension discs in the upper thread path. If you have some *unwaxed* dental floss, you can try sawing that back and forth between the discs. (Do not used waxed dental floss!)
You didn't change the bobbin, did you? I once fixed a machine that was having this problem because the user had inserted the wrong plastic bobbin (one for another machine). The two bobbins looked very similar so it was an easy mistake to make.
If there is a tiny piece of thread caught anywhere in the lower area, that can cause this problem also. I usually take the entire bobbin assembly out and look carefully everywhere to see if I can spot a piece of thread. Check the bobbin case also; sometimes there is a tiny piece of thread caught in the spring tension mechanism where the thread feeds out.
Also check that the thread on top is not getting caught in the spool as it unwinds. I have had it hang up intermittently on the little nick in the spool designed to hold the thread end in place, and that can cause the thread to break too.