Hi,
I don't know if DT-Ints have any impact on DIV32, I'd guess no but I don't know. The manual says that it's not recomended to have interrupts enbles if using ON INTERRUPT because the system variables that DIV32 relies on might get changed by the ISR. But DT-Ints saves the system variables so I don't think it should be an issue.

With that said I don't see the reason for calculating the interrupt frequency "all the time". If I read your code correctly your TMR0 interrupt executes at around 38Hz so it calculates the reload values 38 times per second. As long as the desired frequency isn't changed the reload value for TMR1 doesn't change so why keep calculating it?

Instead, do it in your Main-loop and even better calculatie it only when the desired frequency have changed.

Now, I think you think that you AREN'T calculating it "all the time" but look closely at your TMR0 interrupt service routine:
Code:
' PWM calculation interrupt (Timer 0)
calcint:
' Recalculations
if flag=%1 THEN             '<----Calculate only when Flag=1
   ' Reload timer calculation
  dum=dum1*dum2
  reload=Div32 freq
  reload=(65535-reload)+8
  ' U/F calculation
  if freq<=500 then
     amp=(freq*131)+35
  ELSE
      AMP=65535
  ENDIF
  flag=%0
ENDIF
' Frequency reference control by potentiometer
potsense=((potsense<<6)**maxfreq)+5
if potsense<>freq then freq=potsense:flag=%1
' Frequency limits
if freq<minfreq then freq=minfreq
if freq>maxfreq then freq=maxfreq
@ INT_RETURN
That flag=%1 is NOT part of the IF-THEN statement so flag will get set every interrupt no matter if the frequency have changed or not and therfor the calculations of the reload values will get executed every interrupt.