Your phrasing is slightly ominous, but definitely check your settings for the multiprocessing ("Render Multiple Frames Siumultanously") feature. brokjen up RAM cahce bar could mean that one background instance is not delivering frames as it is supposed to. On that same note, check whether you are not using some plug-in that requires temporal analyses or sequential processing and thus may not work correctly with MP (Timewarp, Lenssblur, particle systems etc.). as a last thing, of course check your settings for the RAM preview itself. maybe you simply set it to only preview every nth framee or are using the secondary options (normally used when using Shift+zero on the numpad).
I am not an erudite; simply an artist with a visual sensibility - thank you for dispensing the myriad info. A artist...Desi.
As Mylenium suggests, the first thing that I would check is what setting you have for Skip in the RAM preview options. If it's anything other than 0, then some frames will not be rendered for RAM preview, so the green line of RAM cache markings will be a dashed line.