Error: RAM Preview needs 2 or more frames for playback

I am using AE CS5 with a Windows 7 desktop machine...12Gb of RAM and an Intel I7 processor...

the short story is that running AE5 sometimes gives me this error and sometimes it doesn't and it is incredibly frustrating to deal with this.
Yesterday my project worked fine..today i get the error message.
Has anyone ever got a real fix for this?? I am fully updated as per update manager.
I have tried restarts but that doesnt do it for me.

Any info would be appreciated!
Thanks
Stan