Are you viewing the playback of the previously recorded images while your system is still operating and recording? How fast are you viewing the playback, 1x, 2x etc?
I am new at CCTV, but have seen this with my system. I have a NUVICO NVDV3 DVR which records while you play back previously recorded images. Sometimes, it will also "skip" or "jump forwards or backwards" over some short segments of recorded video as it tries to read the data and record motion from 5 cameras, especially if I am fast viewing, 4x, 8x, or 16x. However, when I backup the images to CD and play this on my computer frame by frame, everything is there. You could backup the images and view separately from the recorder or maybe turn off the recording mode and trying viewing the playback with no other operations going on. Just a suggestion to see if that is the situation.