Video Oddities

Good day Geeks, I'm running XP Home SP3 and I watch a lot of .mkv video files on a Samsung Home Theater. These files work fine on the computer if they are around 1.5GB, larger than that they get jerky, but that's due to only 8GB of Ram. And they work fine from a flash drive on teh Home theater as long as they are not above 2GB.
Some of these files do not work on the Home Theater however, seems like very large 2.5 to 9GB files get an unsupported file type even though the extension is .mkv. I check the file with G-Spot and even though the extension is .mkv, G-Spot cannot identify and says the container file type is unknown?
Why would a ,mkv file not have a container file type, how do I fix this. Don't want to convert to mp4 because the files are so large it would take days...and obviously the home theater does play .mkv files. I have the Halli MKV codec on the computer but have no way of knowing if its on the Home Theater. si there a way to simply Save the file and create the missing container?