As more camera mfgrs. seem to be using MJPEG (such as Nikon), they are tweaking the rather old CODEC, to improve it. Many cameras, that shoot video with the MJPEG CODEC are stills cameras, that also shoot video.
Below is a re-statement from above - but it is very important, and should be the first course of action, when confronted with the MJPEG CODEC.
In most cases, those mfgrs. are including THEIR version of MJPEG, with their cameras. Their customized version of MJPEG should be on a disc, included with their cameras, though it might not be straightforeward. Their version of MJPEG CODEC might be separate install, or might be installed as part of the camera's driver package, or might be installed with other software on that disc, such as a cataloging, or editing software program. It will depend on the exact camera, and model, but there should be a copy of their version of MJPEG, and that will always be better than a 3rd party version, that will likely be a generic MJPEG CODEC. Check the disc that comes with the camera, and install all software, even if one does not plan on using it to catalog, or edit the footage.