Hank, with regular timers, you need to account for the time it takes to reload the timer after it times out. that what the slider is for. But when using PR, it is my understanding the timer just restarts, so no re-load time, no accounting for interrupt time or ISR entry and exit. It should be right on as I understand it. Are you quite sure your scope is true? can you check the OSC with it? maybe the 16M isn't quite. (Maybe you can put a clock ot on one of the OSC pins if using internal clock)