My heart goes out to those of you who have had such problems. Back in the 80's I had to visit my Singer repairman about 15 times in three months. Years of frustration later, I took a hammer to the machine (really!). This thread has quenched my desire for a mid-arm or long arm. Comments that I have heard from guild ladies regarding their machines include, "I'm past the crying stage, but it's still frustrating." and "Company won't call me back regarding my issues." Such a shame. The reputation for good company service makes a difference to me when buying. I have gone with my second choice product over my first choice because one has a great reputation, and the other, from experience, not so good.
That dealer who says to use only one kind of thread is full of bunko. Absolutely hogwash!!!! My opinion, but also that of Bob at Superior Threads, check out his frequently asked questions. I have used Robinson Anthon, Superior, dollar-a-CONE from internet, Aurefil, etc. on my Juki industrial, and actually I don't believe that I would buy RA again....now I am talking polyester 40 wt thread for zig zag embroidery - not cotton. The dollar-a-cone was stronger than the RA. Please don't dismiss this paragraph, cause I don't have a long arm....the thread is not your problem!!
I wish you the best to resolve your quilting problems.