what may be happening to you is that you mistakenly put the blades on backwards. The liftoff then drop may be caused by getting a small amount of lift from the blades and then when you increase the power the pitch increases and stalls the blades (or they slow down to much from the increased drag from the increased pitch). This happened to me the other day and to many others who post fourms here. Check that if the blades are in line with the helicopter, and your looking at the helicopter from the left side that the one over the canopy has the leading edge ( thick edge) pointing up and that the one of the tail boom is facing down. If not that is your problem.

Hope this helps