The suggestions above are good.
Dump Coats and Clarke thread and get what they recommend at a good sewing machine store.
Sew as slow and as smoothly as you can go -- two layers of T shirt material to check it with moderate thread tension and foot pressure.
Replace the bobbin, needle and confirm the needle size and type, material, foot pressure and thread are compatible. Reset the foot.
Clean the fuzz out of the bobbin and dog compartments.
Give the whole thing a light lube job IAW the manual.