I have a brand new Pfaff GrandQuilter 18.8 and am having a ton of trouble with the tension. Also it makes terrible banging noises. Is it a lemon?

Install a brand new needle.

Remove all the top thread from the machine, ALWAYS RAISE the presser foot and rethread from the beginning (this insures the thread is completely seated in the tension disk). Verify the thread path is correct.

Set the top tension to the midway point (the factory standard), but you may need to tweak it because every machine differs.

Understanding Thread Tension Threads

thread jams or breakage generally points to incorrect thread tensions
read the hand book and adjust accordingly

The free one came in the box with the machine, a replacement can be ordered...
PFAFF Model GrandQuilter 18 8 Users Instruction Manual PFAFF Model...

check with sewvacdoctor.com

check carefully that the top thread is actually gong between the tension plates and not behind them and jamming on the retainer bolt
before threading the needle place the thread up to the point of threading the needle and pull on the thread , proper tension is when there is a slight resistance to pulling the thread
If it takes excessive force to pull the thread then it is caught around something so follow the thread all the way back to the spool to see where the problem is

Many users also reported similar bobbin tension adjust problems. But do you know it can be fixed by yourself? I suggest you to order the following product:
Sewing Machine Repair Guide
It's a professional repair guide written by an expert. Besides PfaFF sewing machine, you can learn how to repair other brands of sewing machine as well. The author claims that you can earn money after you had learned the repair skills!
Good luck.

I don't have the same brand as yours, but on my machine there is a little screw on the bobbin case that you tighten to increase the bobbin tension. If you've got one, turn it 1/8 to 1/4 turn at a time.

My mother has had a Pfaff 1222 for 40 years and loves it. Take yours to a new/different repair shop or find an authorized place for an overhaul. It's not a lemon, just poorly maintained. you have a problem the service people just haven't figured out yet.  If you have taken the same machine to the same people with the same problem more than once and they tell you it's a different cause every time, they are not competent. Also check the owner's manual for any tips.