Thank you to those who have posted describing the problem. I too started to hear and feel the shudders, like going over rumble strips, and had my mechanic drive it and he was unsure of what it could be. So I decided to reasearch it and found this website and the symptons were exacly what my car is experiencing.

After reading the posts I am going to call around to local transmission shops, not the Nissan dealership, and see who has experience in dealing with this type of problem. Unfortunatley my car just passed the 80,000 mile mark, but it sounds like they will not be of use anyways.

Thanks again to those who have posted and I hope that I have caught it in time. My symptoms happened when shifting between 3rd and 4th gear and lasted from 2-6 seconds long. Good luck to everyone who has this problem

Update from Jan 5, 2012: Was able to bypass the transmission to the radiator, did not drive the car and had it towed to a transmission specialist. They replaced the fluid and said it wasn't too bad. Drove the car a short distance to my mechanic, don't drive with the bypass too long or severe damage can occur, he replaced the radiator and everything is going fine as of now. As soon as u feel the shuddering when driving, stop and do not drive it and replace the radiator, it saved me and i thank alll the people in this forum for enlightening me on this problem. Hopefully i will do the same for others.