I think I understand. That's a problem with my machines when there is a little sliver of thread caught in the bobbin tension spring (little tiny sheet metal spring that wraps around the front of the bobbin case), or lint collects around the bobbin assembly, gunked up with oil. The problem cause isn't obvious and is very frustrating. Your problem might not be with the bobbin assembly but the linkage running it. So......
About once a year I haul my machines out into the garage and blast 'em all over with compressed air. Canned air works ok if you don't have a compressor. Then I do what my sister, who owns a huge window treatment workroom in Dallas, does with her commercial machines......floor board the machine to full-tilt-boogie while spraying machine lube/solvent to flush everything out.
I watched one of her service techs do this. I've used WD-40 for this with good results, following up with a drop of sewing machine oil after I dry it all off. Do NOT spray the motor or anywhere near it, if you do this. Use the straw that came with the spray to get good pressure and direction. Spray/clean ALL bearings, top/bottom, underneath, while it's running.
I know this sounds drastic but it gets everything running in tolerance again. It also exposes real adjustment and timing issues if you have any. Sewing machines technically don't wear out but have very close machine tolerances, with adjustments built in to compensate for normal wear. Hidden lint "mud" will turn one into a boat anchor unless you clean it out. Try a mongo cleaning before giving up on your trusty friend. Hope this helps.