What you describe is porpoising. Check your manual, If you hand is off the stick when this happens, you may need to make and adjustmet. If you are pulling back on the stick, then you are causing it. This may be helpful.

THE PROPER USE OF THE ELEVATOR
by Ed Anderson
aeajr on the forums

In flight, the wing does all the work.

The elevator adjusts the attitude of the wing relative to the approaching air.
This changes the amount of lift AND drag produced by the wing. If you apply
up elevator, it raises the front of the wing, increasing lift and drag. If
you have enough speed/power to overcome the drag, the plane will climb. If
you don't, it will slow down, lose lift and "stall". Typically, in a stall,
the plane will pitch up, slow then the nose will drop. This type of stall is almost always caused by the pilot.

Normally, the elevator is used often in flight, to make minor attitude, not altitude, changes. In the right combination with the throttle, you can gain or lose altitude smoothly with the elevator. If you "pull" too hard at too low a speed/power setting, you stall.

When I train new pilots, I often put them into low power settings/or pure
glides with the motor off. Now you really learn what the elevator does.

One of the interesting things is to understand that the wind can cause a
stall. You are flying along and the wind gusts and hits the front of the
plane. The sudden increase in air speed over the wing ( ground speed might
actually go down) causes an increase in lift causing the plane to pitch up.
As that happens, the wind catches under the wing and puts the nose up at too
steep of an angle, causes the plane to lose air speed, causing the stall.

You have to learn to "push" the nose down, back to a more level angle of
attack or the wing will stall. Do this and those gusts will actually help you
climb, IF you keep the wing close to level.

With practice, you start to develop the automatic response that a pitched up
nose needs to be corrected and push it to level to maintain good controllable
flight. I tell them to give it some down for a moment to correct the wing's
attitude. I am not telling them to dive the plane, but to bring the nose, and
the wing "down" to level flight.

The proper use of the elevator is all about keeping the plane at the proper
attitude for the type of flying you are doing and the current power setting.
Learning how to push to level is very important. And, if you stall, DON'T
pull back to try and hold the nose up. Push it to a slight dive to gain
speed so the wing can start flying again.

Don't be afraid to use the elevator. Just understand that you are using
the elevator to control the angle of the wing. The wing does all the work on
the plane but it has to have enough speed to do it. You are there to help.

That was a bit long, but I hope it made sense.

Here are some other tips you might find helpful: Six Keys to Success
http://www.rcgroups.com/forums/showt...08#post3551513