Why does my sewing machine keep jamming?

I have a singer 1120 sewing machine that is now a few years old. I bought it brand new and I was not new to using sewing machines at the time. On the first stitch I tried to do, it jammed. I tried for hours and hours and it would jam without ever completing an entire seam. I threw it into the closet, had somebody... show more I have a singer 1120 sewing machine that is now a few years old. I bought it brand new and I was not new to using sewing machines at the time. On the first stitch I tried to do, it jammed. I tried for hours and hours and it would jam without ever completing an entire seam. I threw it into the closet, had somebody more experienced than me look at it a few years later - and its still jamming. I oiled it as the manual said, took it apart and cleaned it as the manual said, threaded properly, adjusted tensions according to the manual, fabric thickness doesn't matter, it ALWAYS jams and has since day one.

I now feel that it may be a design flaw and would hate to pay a shop to do work on it only to have it come back the same.

Could this benefit from taking it to the shop? What do the shops typically do in these circumstances?

Thanks!
Update: Yes I am holding the thread, threading it properly, etc, etc. I have used other sewing machines many times without these problems. I am more looking for details as to what causes the machine to do this internally that I can check or if a "tune up" would be a good option - or even if I should just purchase a new... show more Yes I am holding the thread, threading it properly, etc, etc.
I have used other sewing machines many times without these problems. I am more looking for details as to what causes the machine to do this internally that I can check or if a "tune up" would be a good option - or even if I should just purchase a new machine.
Update 2: Yes I am holding the thread, threading it properly, etc, etc. I have used other sewing machines many times without these problems. I am more looking for details as to what causes the machine to do this internally that I can check or if a "tune up" would be a good option - or even if I should just purchase a new... show more Yes I am holding the thread, threading it properly, etc, etc.
I have used other sewing machines many times without these problems. I am more looking for details as to what causes the machine to do this internally that I can check or if a "tune up" would be a good option - or even if I should just purchase a new machine.
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