Here's a cause for your HP to get flat:
1. your driving line sucks and you're using much more power than you need to use to get through the corners fast. Also, if your car is setup bad, it may be scrubbing lots of speed in corners as weaponx said.
2. undergearing and overgearing both make motors hot. Undergearing a motor will make it keep a constant high rpm a lot more than normal. A motor draws lots of amps at high rpm, and that makes it hot.
3. have someone show you how to build a motor well
. There is a lot more to motor tuning than cutting the comm and throwing new brushes in. Are you using the right springs for your motor?..ask around. Do you break it in? Low-voltage break-in is crucial to a fast motor. Comm cutting is also a skill that is acquired, if you're not cutting your comm right or not as good as you could, have someone show you the best way.
4. Sure the label on your batteries may say decent numbers, but if you haven't been taking good care of them, they probably suck by now. Make sure you discharge your batteries down to 5.40v for the pack after you use them, or if you dont have a discharger to do that, run them out until the car is crawling (literally).
Hope some of this stuff helps