minecarts seem to lose power going up a long slant in alpha build 7

My minecart rail is from bed rock to the surface and when I ride the minecart up the track it will make it maybe half way up and then stop and go back down to the bottom. (I have powered rails all the way to the top) it is almost like the powered rails don't work going up a slant. it seems like the powered rails at the bottom just push the minecart and half way up it runs out of powers and falls back to the bottom. (this also affects survival)