I also have an HQ Avante. I love it! No problems with the machine at all, but I did have the dealer install it. When they did this, they told me to have fabric & batting ready to quilt and we tested the machine before he left.
I agree with Feline Fanatic. I test the tension often, certainly at the start of every project. As she mentioned, you sometimes have to change the tension when you change thread colors. I think some dyes make the thread a bit stiffer so they behave differently, even same weight, same manufacturer.
Also, I'm puzzled about the diagnosis of your machine being out of time. The symptoms of timing being off are usually skipped stitches, no stitches, or broken needles.
If you quilted a quilt and then saw issues on the back, it sounds like a tension issue with the TOP thread. Almost always issues on the back are top thread tension problems, issues on the front are bobbin issues.
This is how I set my timing:
https://www.youtube.com/watch?v=Q1mRhcquZTM