Override timer interrupt

[Rune Torgersen]

Is there an easy way to use something other than the decrementer for the
timer interrupt?

Reason i'm asking is tha t on our board, the decrementer cannot be
divided to 1khz evenly, so we have rounding errors for time, but we do
have a 1KHz timer interrupt from an FPGA that is source of a T1 clock.

Right now I let the decrementer interrupt do nothing, and made my own
timer interrupt handler that calls the stuff the timer_interrupt usually
does. 

This works, but there are some instability (ie unexplained hangs) that
showed up when I did this.