What is causing the cracks? Is the house racking during winds, is the foundation settling, did the mudders do a crappy job, or....? I'd try to figure out that first, then see if the builder should/could be held responsible. (If any of the 3 options I mentioned are the cause, then I'd say the contractor should be). Bring the contractor back in for a look-see and maybe he/she will have some answers. IMO, you have a valid complaint and somebody did not do something properly. I hope it is just a crappy mud job.