>        bcf     PORTA, 4                         ; turn off the red LED
>        bsf     PORTA, 5                         ; turn on the green led

Hi Rikard, consecutive bit operations on a port is not a good
idea. Potential read-modify-write problems. Try a couple of
NOPs between them

Also, are you using more than one IRQ source ? The comments
and instructions seem to disagree

        bcf     INTCON, INTF                    ; Clear hardware interrupt
flag

        btfsc   PIR1, TMR1IF                    ; Did Timer1 overflow?
         call   once_a_sec                      ; Yes, do our timer-stuff

        context_restore                      ; restore variables
        retfie

For example, you're testing for Timer1 IRQ and branching if true,
but nothing is done if it wasn't Timer1, not even clearing the flag
of whatever it was

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