It was my hood and fan combo when I had the same thing happen. I had to keep tweeking everything until it worked. I ended up with a full Barley Box style hood 2X4 feet with a 6" plexiglass piece across the front at the top. That piece really stopped the dancing flame.
If I stand to torch and lean in too far my forehead hits the Plexiglas.
I run my blower at 1100, 1600 and 2100 CFMS and no dancing flame.
Good luck tweeking as it does take some effort and time.