[Typo3] Problems with Thai-Language

[Butsch]

I created a site in Thai and overwrote lib.stdheader in order to generate 
images as headers. For this I use arialuni.ttf to generate the images. It 
works perfectly well. Except for just one headline which does not appear at 
all. When I copy a working headline into this it works. If I use the same 
signs as before it does not work. Could it be that there is a problem with 
the fontFile I am using? Has anybody expienced this before and knows a 
solution?

Thank in advance,
Butsch