Mettler is a great thread. I love it when I find it on sale, since it tends to be a little expensive. I find that it doesn't make any difference in my stippling if I use the same or different weight on the top and bobbin. I did when I had my older machine, but not now with my better one.
A lot have been said recently about free motion quilting. Some great recommendations out there, but I find that the bottom line is on your machine. Some machines, no matter how much you adjust, practice, and change, will never be able to be used for FMQ. I have a friend that come to my house to use my machine becauase hers will not FMQ (threads break and knots) if she FMQ with flannel on the back. It is OK with cotton. That little estra weight on the fabric makes a huge difference for her.
For years I struggled with an older Kenmore and was about to give up machine quilting, because no matter what I did, it didn't look good. A friend gave me an old Singer that worked great for FMQ. It made me feel better to see that the problem was not just me, but also the machine. I later got an older Bernina that was excellent and with the right machine I was able to practice with confidence and make my stitches look better each time. I am now using an upscale Bernina (given to me, long story), and no matter what I do, my stitches are always beautiful. I am not saying that is all on the machine, but a lot had to do with it.
The thread is very important too, I find that I like to use a higher number weight for the bobbin, because I can fill the bobbin more and when the stitches are looking good, you don't want to stop to change a bobbin. At that moment you want to go on as long as possible without stopping.
Your thread may be breaking due to the tension, the speed, or the threading or your machine. The skipped stitches may be due to the needle. Change the needle and make sure you use a new one. Somehow the needle can not get to the bottom of the quilt to catch the bottom thread.
I hope this helps.
Maria