Usually when the tension issue is on the bottom, it means a problem with the top tension--in the case of too loose/nests of thread, it usually means the top threading is wrong, the take-up tension spring may need flossing (with non-waxed dental thread), etc. But I have to say that when I was having a heap of similar problems with my Singer travel machine, it turned out to be a bent bobbin take-up arm. since it was a drop in bobbin, it required a repair trip.
Recently, mentoring some high school kids, one of the machines was having similar issues and when I checked the bobbin, it seemed to basically lacked any tension pulling the thread from the case so I tightened the case screw and that solved the problem. I know we are trained not to mess with the bobbin tension screw, but you might try that.