Which way does it turn CW or CCW?
Rollover like you describe is generally from not enough rotor speed.
Cures can be reducing the pitch of the blades, tilting the mast back,
or raising the angle that it sits on the ground (bigger wheels maybe).
If you have enough rotor speed in your hand you can generally hand
launch, drop the nose to just about level and add power and it should basically
fly out of your hand with a little shove. You will know that you didn't have enough
rotor speed/power by the quick roll to the ground.
mickey