Girls, sorry to hear you are having machine problems.... bummer!
I am going to assume that you have tried new needles, checked the tension in both the upper tension area and in the bobbin carrier.
When you take it to the repair shop, be sure to take some samples of what you are complaining about, and some pieces of the different fabric weights and battings that you are experiencing difficulty with. MAKE THEM STITCH ON THEM!
Machine dealers tend to have all those lightweight, wonderful precut strips that they use to demo machines with...
The first machine I purchased right of of college was a Viking 6570. (not electronic) This was a workhorse, I loved that machine. It stitched many an hour for me for 25 years. When it was new, I took a class fom the shop, and saved the sample book we made of all the stitches, etc.
Several years later, I had to have the upper camshaft replaced on the machine. when I got it back, the stitch length was way different than it had been originally. The guy at the shop thought I was crazy when I mentioned it when I went to pick the machine up. I marched right back in there the next day with my sample book, showed him that my basting stitch was not nearly long enough and waited while he adjusted the stitch length.
When my sister in law bought her quilting machine, she took some placemats that she had pieced, and spent time in the store, quilting those placemats on each of the machines that they had in the store. She ended up purchasing a different machine than she originally thought she was going to buy, after completing her set of 6 placemats. The guy thought we were totally nuts, but she loves the machine she got.
"Too much fabric, not enough time" Denise