This issue is also happening on 14w26c. A powered minecart will slowly come to a stop after a few seconds of being activated.
I can confirm that this is also happening on 14w25b, and not just corners. It seems to loose power almost anywhere and disconnects with the carts its pulling right when it hits a curve.
Thank you.
Confirmed for 1.8 Release.
Powered minecarts seem to work on straight lines now, but after they hit a curve they will stop moving a few blocks away.