Windows 7 - Custom mkv thumbnails disappear and return later

Hey everyone. I've looked around and not found anything describing my exact poblem.

I recently built a new shuttle system and am using windows 7 media centre.

I am also using the latest shark codec package.

I have been creating custom thumbnails for all my mkv files since my old xp mce system.

Obviously xp never had this issue I'm having but basically:

All the custom mkv thumbnails display fine, but once you play a video, the mkv generated thumbnail replaces my custom thumbnail.

This stays true until I either close mce and open it again or obviously after a system reboot.

In the shark settings I have mkv thumbnails disabled.

My question is, does anyone else have this exact same problem for custom vido thumbnails too?

Is there a solution? I like leaving my system running for weeks, I dont want to have to restart daily because of thumbnails dissapearing and I also don't want to go down the complete thumbnail generation disabling because I want avi thumbnails to generate on their own and mkvs to strictly keep the custom jpg thumbnails and not replace them with generated ones.