Hi Hank,
I think you need to account for the time it takes to
A) Get to the interrupt handler and
B) Reload the timer.

DT-Ints are GREAT but it needs to save a lot of variables before actually executing your interrupt code and that takes time. Once you get to actually reload the timer a certain amount of time has passed. If you don't account for that in your reload value you are basically "turning back the time" so to speak.

Either just tweak the reload value untill you get the desired frequency or try adding the reload value to the current value of TMR1 which should account for any time passed between actual overflow and reload. I think you need to stop the timer, grab its value into a word variable, add your reload value and write it back you TMR1 registers then restart the timer. That means you likely need to tweak the reload value slightly anyway.

HTH
/Henrik.