Sheri,
I don't know if this will help but I'll relate the story just in case. I might have missed a page or two of this thread so forgive me if this has been covered.
I have a Singer 99K I named Rusty. The machine had been in a flood, and was badly rusted. Besides the rust damaged bobbin hook, case, and related parts the thread take up lever had been broken. I replaced all the needed parts to bring the machine up to par but could not get the tension to work at all.
I had replaced the bobbin hook and case and the case latch assembly due to severe rust pitting, that helped but no cure. Then I replaced the pitted top tension disks, a little more help but no cure.
The tension just would not be consistent. Sometimes it would look good for a few inches, then make a huge birds nest underneath.
Finally I got the bright idea to put Rusty side by side with my other 99K, Godiva to see if maybe I could see a difference.
Boy did I. The thread take up lever was not in correctly. When it was all the way up it wasn't up all the way.
When I reset it to match the one in Godiva, the tension problem vanished.
If the take up lever does not go all the way to the top, the thread wont make the lock stitch and the tension will be out.
Since you have had trouble with the position of the TU lever, it might be interesting to compare it to one of your other machines that does sew properly. If the TU lever isn't in the right place, it should be easy to see.
Joe