Galadriel,
I understand how this must be frustrating to you. I am smiling because I too am acquainted with this problem. Be careful if you DO go talk it over with your local sewing machine dealer; it would be too easy to have them convince you that a newer, more expensive machine would be just the trick to solve your problem forever. I don't get angry about such things when they do happen because this is just their job, after all.
I would say, of course, to use the best quality needles you can afford. Where I am living now, that is a joke. Quality is so poor that some of the sewing machine needles don't even have a hole for the thread to pass through! Makes life interesting.
One culprit may be your thread itself and the way it feeds off of your machine. I assume you are using polyester thread for strength in corset construction (I can't remember now if you stated the thread type or not.) The problem caused by twisting thread is exacerbated by polyester, but is not limited to polyester. It can happen with ALL types of thread.
You might try feeding the thread through one of those free standing "cone holders" and then through the mechanism of your machine threading system.
I had another Azerbaijani advise me to "put a nail in the wall over your sewing machine and string the thread over that so that it has a chance to unwind itself and relax before it goes into your sewing machine's threading mechanism. " Needless to say, I did not want to put a nail in the wall of my rented apartment, but I rigged up a support consisting of a broom handle stuck in a huge upside down funnel (to hold the broom handle upright) and put an open hook on the high end of that supported broomstick. The thread passes over that open hook (sort of like a cup hook in the USA) and sure enough, I no longer have the problem. My stitches come out even and the thread no longer gets under stress as it spirals off the spool.
Apparently, in some situations, the thread benefits from doing this regardless of whether the machine is set up to have the thread spool unwind horizontally or vertically.
If you put a nail in your wall and it helps, let me know. (Just teasing, of course.)
Seriously, though, I would like to know if you try something similar to this and if it helps you as it did me.
