Unexpected Microblaze Restart

I have a Spartan-3A DSP 1800 board, with one Microblaze on it. My 
software is witten above the operating system of µC/OS-II. The software 
has only one cyclic task that sends and receives continuously Ethernet 
frames in some specific format and order. EmacLite interrupts are used 
to send and receive frames. The software runs usually correctly; but it 
is often restarted unexpectly sooner or later after its starting.

I tried to find problem in my single cyclic task. But I didn't find 
anything wrong. I wrote a simple program to test my cyclic task. The 
program has no operating system, but a single loop of the single cyclic 
task running once and again continuously with no timer. In this case, 
the single task runs correctly without any unexpected breakup or 
restart.

I also tried to enlarge the stack. But it doesn't help either. The 
problem is always there. Besides, no dynamic memory is allocated and 
released in my single task.

I am really confused what's the problem. Hopefully you could give me 
some hints. Thanks in advance.