Lost MPG thumbnails (again).

I've lost most of my MPG thumbnails. IOW, in a Windows folder in
Thumbnail View, I just get standard icons, not a picture taken from a
frame in the video.

After much googling I tried various fixes, including:
Run: regsvr32 shimgvw.dll but that didn't get them back
Also Run: REGSVR32 SHMEDIA.DLL, which has worked for me in the past,
but no change this time.

Also checked the 'PersistentHandler' registry entries descrtibed as a
solution in this thread, but mine are already OK
http://tinyurl.com/yaw2klq

The aspect that seems different this time is that some thumbnails
*are* still OK. In the past I recall it was probably all or nothing.

Can anyone help me isolate and fix this please?

--
Terry, East Grinstead, UK