Hopper-minecarts will freeze at random areas on the track.

Hopper-minecarts moving on even short distances of regular rails will freeze and refuse to go forward (but can be pushed back the way they came).   Happens in same place once logged in, but place may change after logging out and in again.  Seems prevalent on powered rail-rail interface.  This occurs on powered rails with power applied, on tracks that have functioned perfectly for days. Sometimes logging out & back in fixes the problem.