I know this is really late, but I just now came across this post. I had one of the 750s with a terrible stitch. It didn't matter what adjustments I made, it was just terrible. My little Bernina 350's stitch was nicer than the 750's. There was an update that my dealer put on the machine that helped, but didn't completely fix the issue. The second time I took my machine in to be worked on, it was sent back to Bernina because my technician couldn't get it to work right. By the time I got it back it did work much better, but I still had issues with it. Long story short, after two years of fighting with that machine (and it's various issues) I traded it in for a new machine. I will have to say, I really loved that machine- when it worked right, but I'm a very picky sewer and I just couldn't live with a less than perfect straight stitch. I hope by now there is an update, or fix for your machine, it is a great machine.

Couple of things:
1) oil it up and good - this machine loves loves loves oil. I have a B 780 and oil it (under the stitch plate) ever bobbin or so and the bobbin case itself probably a little less
2) there are a couple of other fixes that the dealer needs to do one having to do with the cover (black plastic piece holding the bobbin basket in) and the bobbin basket itself (the metal piece the bobbin goes into)

So a trip to the dealer is in order
The straight stitch plate helps a lot. There is no doubt about that however oiling and a couple of warranty fixes should help.

Teri

Hello,
A 9mm wide stitch sewing machine will always stitch a better straight stitch if you buy the straight stitch plate to go with your machine. The 9mm machines have a 9mm wide hole in the included stitch plate, to accommodate all stitches, however a straight stitch plate only has a small hole for straight stitching. Because the hole is smaller there is no 'flopping, slopping' of the fabric. (It doesn't get pulled down through the hole as much as normal stitch plate.)
Just make sure that you go into the alert section of your machine and tell it that you have put a straight stitch plate on, the machine will then not allow you to sew a wider stitch and break needles if you change to another stitch pattern by mistake.
I hope this makes sense. I always use the straight stitch plate when piecing and when using the BSR for free motion quilting.
Well worth the expense.
Happy sewing,
Heather

I don't have a 750, but I sometimes see a crooked straight stitch if my needle is dull and needs replacing. It can pierce the fabric so it gets deflected between the weave.

I'd take a piece of the fabric and the thread you are using, as well as a sample, to the dealer when you have the tune-up. Sometimes it's just a coarsely woven fabric.

I bought my 750 QE in August of 2013 and then didn't do any sewing for about a year (long story). I remember reading some posts (may or may not have been here) about the machines having a wobbly straight stitch. So I have just started sewing again recently, and sure enough, the straight stitch is terrible. I can't find any of the old posts to find out what the story was or how it was resolved. I'll be taking my machine in for service next Friday and want to be able to tell the dealer what I've read. Thanks in advance!