2K maxima problem

[bonz2000]

My 2000 maxima has 45k miles on it. We have replaced the o2 sensor recently and now the car drives fine except when going 50 to 60 mph. At that point the rpms jump up to 3000 then rapidly bounce down to 1 to 2000. When I go faster, the car runs fine. When going slower than 50 it runs fine. Using 89 octane, KLM filter, syn oil. No other problems. No CEL lights. Any help would be appreciated.

[bonz2000]

Ps also, we had the transmission serviced this past spring and it checked out fine. Could it be in the transmission? Or maybe in the front end drive area? I also forgot to say that when the rpms go up and down at those speed, the car surges. Tahnks