When taking off from grass with a slightly underpowered plane, try this method.

When you get the plane moving, give it just enough elevator to start getting the wheels off the grass. This will reduce the drag created by the grass. Keep the plane level just off the grass for a little while and let it build up more air speed. Then pull up into you're normal climbout. This will also help reduce the tendency of the plane to turn suddenly after takeoff. It possible that you're trying to take off too quickly and stalling one wing-tip.

Brad