Anyone here adding Thai subs (idx/sub) files to western movies then burning to disc sometimes they show perfectly others get scrambled, all play ok when I use vlc media player but I would rather transfer to disc or my Western digital media player (to get a higher resolution on the telly)

It depends on how new the movie is and the level of English or whatever language that they have. Google Thai subtitles and you will find many to add to your movies. And the other thing is do you understand how to add the subtitles when you find them. I did not and had to be taught properly.