Sounds like you need to run setup on your ESC to get it adjusted to the endpoints on your radio. Read the instructions that came with the ESC, and they should walk you through the process. Also, you'll want to read the instructions for your radio for fine tuning that as well. Lastly, there is a possibility that your battery pack may not be supplying enough voltage to the ESC which is causing it to glitch or hesitate. Try upgrading to one of the newer NiMH packs with either GP or IB cells.