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context switching/timer handler

hello,
I am designing a preemptible kernel with the help fo oskit and running
into some problems with the timer_handler/context switihing.
below is the code for the timer_handler....
-------------------------------------------
void
timer_handler(struct trap_state *ts)
{     
  chronon++;
       
if ((started_init) && ((chronon % 10) == 0) && !IN_KERNEL) {
    if (curthread != IdleTask) { //curthread is the thread running when
                                     the interrupt ocurred

      curthread->eax = ts->eax;
      curthread->ebx = ts->ebx;
      curthread->ecx = ts->ecx; 
      curthread->edx = ts->edx;
      curthread->esi = ts->esi;
      curthread->edi = ts->edi;
      curthread->eip = ts->eip;
      curthread->retaddr = ts->eip;

      out_thread = curthread;  
      Dispatch();              //Dispatch sets the new value of curthread
			       //a global variable, according to
				//the scheduling policy.
      ts->eax = curthread->eax;
      ts->ebx = curthread->ebx;
      ts->ecx = curthread->ecx;  
      ts->edx = curthread->edx;  
      ts->esi = curthread->esi;  
      ts->edi = curthread->edi;  
      ts->eflags = curthread->eflags;
   

     in_thread = curthread ;
    }
    ts->eip = (unsigned int) do_switch;
                            //do_switch saves and restores the stack
			    //	pointer esp which is not done by oskit    
 
  }   
   
  return;
}
---------------------------------------------------------------------------
the code for do_switch is as follows:
Note : esp and ebp are at offsets 8 and 12 in the struct thread.......
and the return address at offset 40... 
-------------------------------------------------------------------------
        .text
        .align 4
        .globl do_switch
        .globl in_thread
        .globl out_thread

do_switch:
        cli
        movl out_thread, %eax      //stores the old context in eax...
        movl %esp, 8(%eax)         //saves esp
	movl %ebp, 12(%eax)        //saves ebp
        movl in_thread, %eax       //stores the new context in eax
        movl 8(%eax), %esp         //sets the new esp value
        movl 12(%eax), %ebp        //sets the new ebp value
        movl 40(%eax), %ebx
        pushl %ebx                 //pushes the new return address on 
				  // the stack
        movl in_thread, %eax
        movl 20(%eax), %ebx        //set the eax and the ebx values.
        movl 16(%eax), %eax  
	sti
        ret                       //return to the return address of the 
				// the thread in_thread

-------------------------------------------------------------------------------
We plan to run some vision applications on the kernel.
On running an object tracking application (for which we use the bttv
driver ported on to oskit..) ; the kernel inexplicably crashes after 
about a minute.....
The call to Dispatch() also occurs in all the system calls....
On disabling the timer_handler (ie simply returning from
timer_handler() without doing anything)....the application runs fine
(context switching happens during system calls like sem_wait etc.)

any pointers on what could be wrong will be appreciated.....
thanx in advance

Puneet