I think the greatest source of your troubles is poor support and service more than anything else.
FWIW, your Kenmore 19110 is actually made by Janome, and there is a
Janome labeled machine which is identical.
Anyhow, I do understand how the upper and lower tension assemblies function from a mechanical perspective. In crude terms, there is spring that holds the discs up against each other which in turn, squeeze/pinch the thread to create the tension. A threaded nut or knob is turned in or out on a threaded rod to increase the pressure of the spring against the discs. The odds of this spring getting loose or the nut turning after you sew a few inches is pretty low. I would suspect that something else is the source of your woes.
This is were a good shop is a valuable asset. They can sit down with you and the machine and figure out if the brand or type of bobbin that you are using is incorrect, if there is a upper or lower threading error, incorrect needle type, bur on the thread path, perhaps the thread cap isn't being used, etc... I can't really venture a guess as to exactly what the issue might be based on a description of "wonky", but I really doubt it is a serious mechanical issue with the machine. I'm not disrespecting you in any way, just pointing out some of the technical/mechanical background and that it's often the little things that bite us all in the rear end.