I have a 2007 Impala SS, and experienced the similar problems. Around 78k miles the transmission started having trouble engaging in first gear. It didn't happen all the time, but when it did, you could feel it slam into gear. I notified Freeman Mazda in Irving, TX, where I bought the vehicle used, but got no response. Over 60 days from the purchase date, the problem got worse.
I took it to the Service department, and was told that there were no computer codes, that there was no check engine light. I swore up and down that at 40, 60, and 80 mph, the transmission would shudder and the rpms would fluctuate wildly. The first go round, I was told they were unable to "replicate" the issue. On the second day of being in the shop the manager confirmed the problem, but was told there were still no codes and that I should send it to a Chevy dealership to have it looked at a second time. Eventually, the transmission failed at a stop light. I hit the gas, and the car went nowhere.
Luckily I had an extended warranty, and am currently having a new (reman.) transmission installed. Other than that, its a great ride. Black, Black leather interior, tinted windows, etc...
