playback stops mid timeline

[super gnu]

when i hit play to cache a sequence, i go away, and when i come back, its stopped on a frame somewhere in the middle of the sequence.  if i go back to frame 0 and hit play, it smoothly plays until the frame it stopped at, then continues caching the subsequent frames. 

its not reached the limit of my ram or anything (cache was at 13GB, ive got 64..) 

any ideas why its doing that?