Hmm it looks like your texture pack is massive with 394 512x textures. It just barely fits into a 16384*8192 atlas texture, 1 more 512x texture and it'd have to double. That's just for the block textures.
FastCraft duplicates the texture atlas for double buffering which apparently exhausts your GPU memory, making the driver re-upload the texture every frame.
I'll add a config setting to control the duplication and probably some heuristics for the default behavior, it's obviously better to optimize for memory with very little GPU RAM or very high resolution textures.
Edit: LloydSev's case seems similar.
The png files in your texture pack are very efficiently compressed, the textures on the GPU are not. Even if it'd fit all into the GPU memory it's still up to the driver how to allocate everything. You could try to run FastCraft 1.6 with a texture pack which has about twice as many large textures for both blocks and items. If this causes similar issues my theory could be verified.
I went ahead and replaced the erroneous 512x textures in my pack (just one mod's textures), with 64x textures and the issue resolved itself.