I use the techniques in this video to set tension:
https://www.youtube.com/watch?v=Q1mRhcquZTM
although I cannot loosen my bobbin thread as much as he does (bobbin screw falls out!).
Since it's just an occasional "blip", it really sounds like lint in the bobbin to me. The video shows how to clean the spring in the bobbin case. Second-most likely cause for me would be inconsistent speed.
What kind of setup are you using? Sometimes people have tweaks for specific machines.