We bought a 2005 Buick Lacrosse in 2006, brand new. It just turned 50k miles, and has been very well taken care of. Recently while trying to accelerate there is a strong hesitation, like the car doesn't want to shift into gear. Sometimes it will knock or clunk once it shifts. The last time it did it, I noticed the rpm needle was bouncing up & down. I am going to take it in tomorrow, but was curious if someone could give me some suggestions as to what it is. The car is paid off, so hopefully it's nothing major 