I would still complain to the company. maybe they can advise you if not replace it.

You didn't tell us if you were using the same thread as in the store? Same thread top and bobbin? Some machines don't like some threads and some don't like different threads top and bobbin.

If it works fine at the shop and not at your house I would be looking at the
power. Is your sewing machine on the same circuit as other appliances?
Or it could just be your foot pedal that's worn out.
I'd make sure the power is ok before buying another machine.

^ This ^, please. It would really help so folks could understand what you mean by "bad stitches".

Ideally top and back of fabric, too, as that shows what the needle thread is doing vs the bobbin thread, as well as the feed dogs.

Hmmm. I also would be suspicious of the power in your house. Can someone from the power company come and check where it connects to the house and the pole or underground?A friend of mine kept "burning out" dishwashers and microwave ovens. Complaining about bad brands. I suggested the power company. They did come out and found the line to the house was rotted almost all the way through. Once that was fixed, everything else worked great.

Also ... whether they used your power cables and foot pedal at the store?
It's quite common that they use their own, for simplicity of hook up ......

Have you stitched on it at the store, yourself?
Two things ... maybe something different as to what you are doing to what the service tech does?
Or perhaps it takes a certain amount of time for it to act up?

It's the same thread as what the store used when they fixed it. in fact, the bobbin was still full from when the dealer was sewing with the machine. I have tried everything that all of you suggested with no luck.I am ready to pull my hair out!

I understand well what you are saying. I discovered along the way that if the quilt does not have the proper distance between quilt and the throat of the machine, especially if the distance is to high it will mess up really bad. You may want to adjust your roller bars some.

QuiltE I am just leaving now to go to the store and have the repairman look at it. if it sews ok there I don't know what they will do. I brought a sample of all the stitches I tried at home with me to show them, I'll let everyone knows what happen next...Thanks all

Check your shoes! Now I'm not making fun, but you could have a magnet near the machine or your finger rings may be doing something. Something sounds odd. Is there a magnet near on the table or scissors, or just anything? Barny