Interrupt problem

Using the STR912, I am using several different interrupts (usb, uart, timer and wiu) all of the irq handled as VIC (vectored) with different priorities (2->6). It happens that something my firmware hangs and I think is because once I get inside a interrupt the IRQ are disabled (according with the datasheet) and if other IRQ occurs during this time my program crashes ... no Undefined_Handler IRQ, no Abort_Handler IRQ ... nothing.
The expected behavior is that if not working with nested interrupts the execution should finish the execution of the actual interrupt and then switch for the next one, even if an interrupt occurs during the execution of the previous (atomic execution), right?
Someone has an idea what can possibly be?

This is basically how I use the IRQ handlers:

void TIM1_IRQHandler(void)
{
    // Clear TIM1 counter
    TIM_CounterCmd(TIM1, TIM_CLEAR);
    // Clear TIM1 flag OC1
    TIM_ClearFlag(TIM1,TIM_FLAG_OC1);

    // whatever I want

    VIC0->VAR = 0xFF;
}