I know this may seem like a small deal. But carefully check the bobbin area. Especially where it is going through the part that the thread is pulled through.I found a picture to show the area I am talking about. Tiny pieces of lint can get caught in here and mess up your stitches. I found this out when my Brother PE 770 started messing up stitches. It saved me a $150.00 service call.
Also if you can check the whole threading route for any lint, I use unwaxed floss to do this.