My GZ250 gets funky in fifth...

[Cordell_Love]

So this is my first bike and I've been ridin for about two weeks. I love my lil GZ250 but I have a problem. The bike runs great except when I shift it into 5th, then bike lunges or lurches rhythmically every 5 to 10 seconds. You can hear the engine revving up before it lunges too. It only lunges in 5th and only if the throttle is open. Weird huh? Thanks to everybody for any and all advice on the problem.