Hi,
I think what Mark is saying is that there are parts of DT-Ints missing in your code, you don't seem to include the needed files and there's no place where you're actually defining/setting up the interrupts. Is that really the complete code you've posted? My guess is no since I don't think it would even compile like that.

What's your oscillator speed? 8MHz? Do you have that defined somewhere in your code?

I'm pretty sure that interrupting at 4us intervals isn't going to work, IF you're running at 8MHz that's only 8 instructions between each interrupt and it takes WAY WAY more than that for DT-Ints just to do the context save and restore, the interrupts are "running over them selves".

/Henrik.