System information? Comp settings? What version of AE? MP settings? Audio settings? What footage is used? You need to be much more specific.
The nMPr with latest version of AE on os x 10.10
32Gb of Ram 4Gb for background
MP settings doesn't make a difference
Footage doesn't make a difference
Comp doesn't make a difference
I have this same issue and it's been doing my head in!! I thought I was the only one. Also seemed to start occurring around last update.
As Niel experiences, footage/project type makes no difference.
Checked all settings and can't see any logical reason why it renders the first 25 frames (at 25fps) but then plays from current time+25frames. It only plays the skipped 1 sec when the RAM preview loops play back to the start,
Known Yosemite bug. There are a bunch of threads on this problem you can find with a search