How long have you had the car? There are time limits to the buyback procedue GM uses. I used to be a service manager at a dealership that was part of GM. We did a few buybacks, but normally they were for larger issues, more mechanical issues. (not that your issue is any less annoying or less important, just less severe in their eyes)
If GM is willing to fix the issue and are working on the problem, that is probably all they will do for you at this point. I know it usually takes time for them to come up with a fix for the issue. Normally that means RE-fabricating a part of the vehicle, or retro-fitting the part in some way, and that takes time. It doesnt mean they are 'making up' a solution, just re-manufacturing the part. They want to be sure that they can repair the issue without having you come back even more times than you already have, so yes...it may take some time.
You can try the lemon law if you qualify. Otherwise work with GM and try to get them to offer you some compensation instead-possibly an extended warranty or something like that. Maybe even free service visits/oil changes for a specificed time period. Something to compensate you for your time and dissapointment with the issue. If you dealership says no, call GM. Ask to talk to/meet with a field service engineer. They have people in each part of the country (engineers who basically designed the vehicles) who will meet you at the dealership to evaluate your issue first hand in most cases.
Sorry this got so long....just trying to give you some options....Hope you get a solution that works for you!
OH, and keep all the paperwork from ALL the visits you have taken it in for the problem. Even if they dont want to 'write it up' each time you go in...MAKE them. It is your only proof they have made an attempt to fix it if you decide to use the lemon law.