Instant Interrupts - Revisited

[bcd]

I have been playing with the code shown below on a 16f737 as I was trying to get different prescaler values into the int routinues. As part of the testing I stripped the code right back to just be a simple interrupt with a value loaded into the pre-load registrer, but I seem to end up with an interrupt time of 204uS. According to the Timer Calc in Mister E's PicMulti Calc I should get an interrupt every 5.019mS. (see attached image for capture of the output waveform - I love my PICkit 2!)

This code is lifted straight from Darells example with the blinky led (1-2-3 version), and all I have done is add the pre-load. I have also removed the pre-load and still get the same time for the interrupt.

DEFINE OSC 20

rs_led      var portb.7 ' rs232 led
led         var portc.4 ' was Data_Out line

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

INCLUDE "DT_INTS-14.bas"     ' Base Interrupt System
INCLUDE "ReEnterPBP.bas"     ' Include if using PBP interrupts


ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
       INT_Handler   TMR0_INT,  _doBAM,   PBP,  yes
     endm
    INT_CREATE     ; Creates the interrupt processor
ENDASM

OPTION_REG = OPTION_REG & $80 | 1     'Set TMR0 Prescaler to 256, leave RBPU alone

TMR0 =  158 ' preload with 158 to give 21uS interrupts (at 1:1)
' this should then give Interrupt every 5.019mS at 1:256

@ INT_ENABLE  TMR0_INT     ; enable Timer 1 interrupts
'~~~~~~~~~~~~~~~~~~

goto main      ' jump over ISR

'---[TMR0 - interrupt handler]--------------------------------------------------
doBAM:
toggle rs_led
@ INT_RETURN

'~~~~~~~~~~~~~~~~~~~~~~~~~~~
 high rs_led
main:

'''''''''''''''''''''''
' do some flashing led stuff
'''''''''''''''''''''''
high led
pause 500
low led
pause 500
goto main

There has got to be something really simple I am missing...
bill.