Can't stay in gear
 

While doing some riding i noticed that the rear darraileur can't stay in certain gears.
The problem is that i select one gear and often it keeps chanching between near gears.
I did some searching on what could be causing this but to no avail.
What do you think could be happening?
Thank you.

If your shifters are indexed autoshifting is usually an indication that the shift cable needs adjusting. If your shift cables and/or cable housings are old or dirty they may need replacing.
If your shifters are friction then you may need to increase the friction at the shifter.
You may find helpful information at http://www.sheldonbrown.com and http://www.parktool.com.

Al

This helped me: http://bicycletutor.com/adjust-rear-derailleur/

The shifter could be bad also. Jack