Alt hold

When ever I use ALT HOLD, the quadcopter will rise even though I am not trying to change the altitute.

I fly up to 20 - 30 feet and set ALT HOLD. Then as I fly around it climbs.

Any suggestions?

you can manually set the height change with your throttle stick after alt hold.
may you try to set the mid-level of thrust (there is a automatical calculated hover-thrust) and enter this value in the mid thrust.

so if you change from stabilize to alt hold, you are perfect in the middle with your thust stick, and not in lets say middle +10%, because than it will rise automatically, because you are ordering it too