The past two times I've driven in the rain I have experienced something odd. Around 60 mph I've felt a jolt - similar to the feeling of the speed regulator kicking in. Like I said, it's only happened in the rain. Both times were long-distance trips (100+ miles) and possibly during shifting. At first I thought it may have just been losing traction in the rear wheels, but I'm not so sure anymore. I've had the Xterra since 35,000 miles and now it's at around 110,000. I'm embarrassed to say that I cannot remember if I've had the transmission fluid changed the entire time. Any ideas?

Cruise control on?