What format is “undf”

The chances are that when you were recovering them tiny bits of the code was eater not there or left behind. This will make the video corrupt and it will be almost impossible to repair the code, even with hours and hours of work the most you could get would be able to get would be a few seconds in total, and these will not be in one piece. Most of them you wouldn't even be able to recover at all. Sorry but I think you may of lost them unless some one else has any suggestions.

You could try this link

You are going to find some slowdown on a machine running Windows 7 with only 2GB of RAM, a Pentium(R) Dual-Core T4500 @2.3Ghz and an Intel GMA4500M when running high-quality, heavily compressed video. The reason you're able to play normal .mkv files more smoothly is likely to do with the level of compression. This from Wikipedia:

In comparison to AVC, HEVC offers about double the data compression ratio at the same level of video quality, or substantially improved video quality at the same bit rate.

So because your HEVC files are more compressed, your computer is having to do a lot more work per frame to uncompress everything.

What I might suggest is uploading these videos to something like Google Drive and watch them from there, letting the cloud servers do the hard work of decompressing. It's maybe not the best possible solution, but it's at least one solid workaround, and it's worked for me when I had similar problems in the past.