You haven't mentioned what long arm machine you have that your having tension problems with. Do you have a "Towa Bobbin Case Tension Gauge" to help you with the tension in your bobbins? If not I recommend getting one (they aren't cheap around $70.) but they sure help take the frustration out of setting the perfect tension in you bobbin case.
If your getting loops underneath I am willing to bet that your bobbin tension is way too tight & your top tension might not be tight enough. If your getting loops on your curves underneath you are probably going way to fast on those curves.
So before getting rid of your machine get a Towa gauge, there are two sizes, most long arms take a "M" so check before getting one. Also make sure that you have the correct needles for your machine. Different machines take different needles.
Good luck & don't give up.