That is a bit wierd, because what I posted should've had a freq of about 36khz (73.5khz int rate, 20mhz/4=5Mhz instruction rate / 68 = 73.5khz toggling the pin).

If anything, the freq should've dropped BELOW 36khz due to not reloading the timer with 'fixed' values and not compensating for the extra cycles in the interrupt loop.
I get the feeling that the 'scope isn't being read right (i.e. only measuring half of the square wave, not the whole thing)

And yes, I agree completely about DT_INT's vs. ON_Interrupt.