I have a 2006 Grand Prix GT and it has 214,500 miles on it. The tranny and engine are original and the only thing I have done to the transmission is fluid changes and I also replaced the pressure control solenoid about 7k miles ago. Now, the transmission shudders between 1st and 2nd gear when up-shifting around 4k rpms and higher only if the fluid is cold. Once the transmission gets up to temperature (around 174 degrees F) the shift is smooth. It shudders every time when not up to temperature but again once its up to temp, the shudder goes away and it shifts smooth. I've used bottles of fluid to clean the clutches and insides of the tranny a couple of times, most of which only act as a band aid. Does anyone have any idea what could be causing this shudder when the tranny is cooler?