Agree on the bunching. Other advice, is your chair adjustable for height? Maybe your seating position could be improved. Also, try and get some surface to your left and behind you to support the weight. I have a little chest of drawers on wheels the same height as my sewing table that I pull around when doing a biggie. And try plan the quilting so you only have half the quilt through the throat at the most. I sometimes grab a fistful of quilt with my left hand rather than doin the flat hand thing and it works fine for me, as long as the area immediately under the needle is flat it won't pucker. Hope it helps. It's a pain to have to suffer for your art and MQ is such fun! Good luck.
Sorry to repeat some of the advice above - think we were posting at the same time! :-D