I'm sorry I don't know your exact machine but I understand the frustrations!
Did the machine ever work correctly for you? I know you have already done the obvious but when I have the sort of thing where "it was all working just fine the other day" that a really thorough cleaning of the bobbin casing and general area can magically make things better. Even a new needle can have a flaw, or we can create the same flaws by our actions and motions.
I describe my Bernina as cranky and notice that it seems to really be impacted by humidity changes. There are days when I can just sit down and sew (which is what I expect) and days when that is simply not going to happen. Even when all I want to do is straight stitch, sometimes I work with my zigzag to get the tensions right for that, and then switch and see if it holds for the straight stitch.
What is the fabric/project like? I find I get a lot of skipped stitches on batiks and finely woven fabrics and get better results with a microtex point, other than that I use universal needles but do go up to 14+ for the quilting. This year I have a lot of tops ready to go and I will be trying different types of needles to see if I can tell much difference.