I have a 758 also. Mine does the same thing unless I use the original bobbins that came with my machine when I bought it new. The ones you buy today, both singer and the after market with the white bottom do not wind properly and do exactly as you describe. No problem with the original ones here. You need to wind bobbins on the slow speed not fast. There have been several threads here about 758 bobbins try to do a search to see if anything in one of those threads might help you.