Cutting off at half throttle is almost certainly your batteries. You could try to use a glitch buster
for less than $10. Most likely your batteries are getting drained to the point where your receiver isn't being powered and shuts off.
Although that may eliminate some or all of your problem, you are going to need some better batteries. If half throttle is enough to rob your receiver of power, you are most likely putting a tremendous strain on the batteries. I bet they are nearly smoking after a run? They won't last long like that!
Go lipos if and when you can afford it. A pair of lipos isn't much more expensive than a pair of quality nimh packs today, and the performance and longevity is much better.
As for the setup, not much to be done. Set your throttle and brake end points at 100% before calibrating your ESC. Leave them set at 100% unless you have too much brakes and want to turn that end point down, although often times adjust the end points can create problems with your esc reading properly. Set your steering end points just under the point where there is binding. If you want less steering you can turn down those end points without issue.
I'm not familiar with the DX2 but I do use dx3's and I think they have the same options. You can smooth out your steering and throttle using the exponential settings. Negative exponential will make the steering or throttle less sensitive around center, taking some twitch out of the steering or overpowering out of the acceleration. Personally I prefer to leave those alone and learn to drive the transmitter. It feels more predictable to me like that and you may need to have full response on occasion.