[deepskydiver]

Running the current Krypton version on Windows 10 on an Intel NUC.

I have a couple of problems, some xvid files regularly crash Kodi and mpeg2 TV recordings take seconds to skip forward or back. 

Are the decoders built into Kodi? How do I go about diagnosing and fixing / updating to fix any problems?

[FernetMenta]

Kodi uses ffmpeg for decoding. xvid has always been known to cause trouble with hardware decoders. On Linux we allow to switch off hw decoding for specific codecs, not sure about Windows.
TV recordings are mpegts. This container has no index, means searching to a specific timestamp is done via bisecting. If your files are on a network share, seeking can be slow. But in general it is not. I suggest to try v18 because it has fixed bad SMB performance.