Just got back from work, and as I was driving between locations my Maxima did something odd. I was pulling away from a set of lights when it quickly dropped in revs to about 1000 then jumped back up again to about 1500. It's never done this before, and I wasn't driving any different to what I usually do. What likely happened and is it bad (or a sign of bad things to come)?

Thanks for any help

 

Need a bit more info.
Speed/gear etc

 

Was going around 50km/h, was accelerating up to 60km/h (roughly 31mph accel. to 37mph). I'm pretty sure it was in gear two at the time (it's an auto ). It made a sort of groan noise at the time, kind of like changing gears but had a strained tone to it. Hope this makes things a little clearer

 

Yeah, that's probably not a good thing, how many miles on it? So it was sorta like it wanted and started to go into first but instead went into second, abruptly ?

 

It's got around 160 935 miles on it. I think it was more it was in 2nd, started going to 3rd then quickly decided to go back to 2nd again. It didn't jerk or anything when it did this, just made the noise and the rev fluctuation. Any idea what could be happening? What should I be doing now to stop it from happening again?

 

Just had another thought. About 2 weeks ago the car went in for a warran of fitness. In the report they said the alternator belt needs to be replaced soonish as it was showing signs of cracking. I believe the cam belt also needs replacing soon too. Could these have caused it?