I have the Janome MC9700, and have had the same problem. Janome's as a class can be very "thread fussy" and "bobbin fussy". However, there is no reason you shouldn't be able to use Gutermann thread in your Janome. And there is no guarantee that a different thread will produce better results, so don't let them sell you on that.
Do make sure you are using bobbins meant for Janome machines. They say you can use "almost any" kind of bobbin. I'm here to tell you, "almost any" kind of bobbin doesn't work so well. I stick with bobbins specifically made for Janomes, and it helps a lot.
If you still have any free lessons left from your purchase, I suggest you take your machine back to the store, ask for someone who teaches sewing (not a tech) and is familiar with the machine, and explain the problems you are having and ask for suggestions. You probably should make an appointment for this. Make sure you have them go over in great detail how to thread the machine and bobbin. Then make them watch you practice until they're sure you're doing it right, even if it takes several times. I finally figured out that a small threading error was causing part of my problem. Or sit down with your manual and go over threading, again and again. Make sure you're doing it right every time. This is no small thing. If you've only had it since October, you may not have the full "hang" of threading, especially if this is your first Janome. It took me several months to figure out what I was doing wrong. Sometimes I''d get it threaded right, and sometimes I obviously wouldn't. Sometimes I'd get that "THREAD ERROR" signal, and sometimes I wouldn't. I'd just have thread breakage or bird's nests under my fabric. Also make sure you are winding the bobbin correctly. Again a small thing, but an improperly wound bobbin can cause all kinds of problems, and every machine has its "secrets" for doing it correctly. No disrespect meant. I've been sewing since I could sit on my great-grandmother's lap while she worked the treadle. Every machine is different. I've made my share of mistakes with my newest "computerized" machine. I had lots to learn, because there was lots to learn. What you're getting here is stuff I learned the hard way!
Make sure that the thread you're using on top is the same thread that is in the bobbin. I don't know why, but Janome's seem to require this. Every time I tried to use a pre-wound bobbin, I had bird's nests. You can still use different colors in the top and bobbin, if you like; just make sure they are both Gutermann threads. I hear the same thing from lots of other Janome owners who have owned their machines much longer than I have. In fact, I received this advice from a quilter with 3 Janome's who'd had one or another for over 20 years, and said they were all alike in this regard.
If all else fails, check the bobbin tension. That's right, the "third rail" we're not supposed to touch. My instructor at the shop I bought it from showed me how to do it, even though she wasn't supposed to do it, but she showed me that my tension was "off". How? By cutting a square of fabric, and sewing a straight stitch across the bias -- you know, the part of the fabric that has a slight stretch to it -- and then giving the fabric a couple of sharp snaps across the stitching. What happened is that the thread broke. She says that means the tension is too tight. If the thread breaks on the top, the tension on the top is too tight. If it breaks on the bottom, then the bottom tension is too tight. In my case, both broke. So we played with it. Loosened the top a bit. Then went after the bottom. There we unscrewed the face plate and lifted it up a bit. That exposed the bobbin case, and Lo and Behold, right there in the front was a small screw right on the bobbin case. I've been sewing a long time, and this screw looked exactly like the bobbin tension adjustment screw I had on the machine I had 30 years ago! Honest! And you adjust it exactly the same way. I used a tiny screwdriver I took from an eyeglasses repair kit to turn the screw. As you're looking at it, turn it 1/4 turn to the right to tighten the tension, or 1/4 turn to the left to loosen the tension. Never turn it more than 1/4 turn at a time. Just small increments. Then put the face plate back in place, but don't bother screwing it down in case you need to make more adjustments. Do the bias test again with the straight stitching, give it a snap, and see if anything breaks. When it gets to the point where nothing breaks, you tension should be perfect.
I say "should be perfect" because of course you have to look at the stitches too. Are they neat and snug against the fabric on both sides? Is there any top thread showing on the bottom, or any bottom thread showing on the top? Shouldn't be. If there is top thread showing on the bottom, your bottom tension is too tight, and possibly your top is too loose. If there is bottom thread show in the top, then your bottom tension is too loose, and possibly your top tension is too tight. Just keep working away at it. It can be painstaking. If you see thread that loops below the fabric, then the bottom tension is definitely too loose. It needs to be snug up against the fabric, and the top thread should look like the bottom thread. I really don't know why the sewing machine people think we're too dumb to deal with bobbin tension. Mine was so loose, it really needed to be fixed, and it needed a quilter to recognize it. The techs who clean and repair the machines don't know anything about tension.
If you get all of that fixed, and you're still having bird's nests, something is still wrong, and you need a Quilter familiar with your machine to help you, not a repair person. In the short term, you can start every row by pulling up the bobbin thread on the first stitch -- pull it right up to the top of the fabric --- and that will prevent bird's nests.
Hope this has helped some. Keep at it! You have a lovely machine, and there is an answer to this problem! And it's not the thread!
Warmest Regards,