N750 choppy video play back on avi but not mkv

I recently set up the N750 and have a few WDTV’s set up throughout the house accessing a MBL connected to the router, but when I play tv shows specifically lower resolution files from the MBL to the wireless WDTV the video starts out choppy and will never catch up and play correctly. Yet I can go access a 720p mkv movie from the same MBL on the same WDTV wirelessly and it will play flawlessly from the beginning. Does the new router limit what type of video play back it uses the fastrack technology for or is there a setting I am missing to make this work correctly. Thanks

If you try to play the same lower resolution file from a USB memory does it plays fine?

Well I have a WDTV Live Hub that is a wired connection and plays the files correctly and all firmware is up to date on both devices so I am assumming it would as well for the WDTV Live.