My bike will/wants to switch gears while im riding it. Why?

I had all the cables replaced a few months back. Ive done a few hundred miles... when im riding it will start to change gears.. How can I fix this? Its a older K2 proflex. thanks My bike will/wants to switch gears while im riding it. Why?
The new cables have stretch and need adjusted. Watch this tutorial, towards the end it will tell you how to adjust it.



http://bicycletutor.com/adjust-rear-deraMy bike will/wants to switch gears while im riding it. Why?
Sounds like you have an automatic shifter.. Take it back and demand that you get a manual transmission.
Very likely, the new cables have stretched a little and need readjusting. This is quite normal, though many mechanics will attempt to stretch the cables after installation to prevent this.
This is normal after installing new cables. After installing new cables they wear in and stretch some, a simple fix is to make the adjustment using the barrel adjusters.

Here is a link to help you with the adjustments.

http://www.sheldonbrown.com/derailer-adj
new cables stretch over time. it is a simple adjustment. back the cable tension barrel out (on the derailer where cable goes through). turn it counter clockwise about half a turn. try it and see. if it still does it, go a little more until it stops autoshifting. i suspect by the time it gets to this point you actually need quite a bit of adjustment. if you are unsure, bring it back to the shop and have then do it up. it is a good thing to learn though, because this is something that needs done from time to time.
GS is right because i been there and took it to a bike shop and they said that the cable is stretch that when you ride your gear is not in fro example 5 but 5 1/2 so there for it goes back and forth from 5th the 6th gear hope i help
the cables streched, just bring it back to ur local bike shop the might adjust them for free