The whole Chunk does not disappear, only parts of it like the lower/ upper half of a tree or dirt (where i can look all the way down to the world hole). They reappear untouched.
I allocated 1GB of memory for Tekkit. The server i'm playing on is a homer server by my friend but this bug does not only appear in tekkit smp but also in technic ssp and tekkit ssp.
No, the movement and crafting works fine for me.
I'll gladly provide you with more information if needed.