You would have to test the battery under load to see what kind of voltage drop you are getting. You said it's happening with both the steering and throttle, so it shouldn't be any kind of binding issue. You could try buying a better pack and see if that resolves your issue. Or maybe going with a...
What kinda pack is it? When batteries get old, they will experience more voltage drop under load sometimes. This causes them to have a lot lower voltage than the battery has with no load on it. If it is a lipo, check the IR on each cell to make sure you don't have some high resistance causing...