i recall seeing similar issues when i converted compressed tgas to dds (or the other way round, i don't remember). anyway, the tga/dds converter can only handle uncompressed TGAs, otherways the result looks like a striped rainbow colored sweater. not sure if this is the case here, and somehow i doubt TAD would fall for such a beginners issue, but maybe it's a start to look for a solution?
Before I compress tga images to dds I use tga flipper twice. I remember reading in one of the forums (long time ago) that beside turning a tga image upside down (hence use it twice so the images are the right way up), it is also supposed to remove rle compression.
@Pstemarie What do you do when your textures have transparency? That's what the extra 8 bits in 32 bit textures is for e.g. 8 bit each red, green, blue, alpha.
TR