Thanks for the direction Darrel. I did some research on the USART and like what I found now that I'm beginning to understand the use of your interrupt routine and how the information is presented in the data sheets. I think it makes more sense as the need increases.

7812 baud ... ECU / ALDL ??
The 7812 baud is on a motorcycle ECM alias ECU.

The HPWM command doesn't like being updated continuously.
It resets the PWM cycles every time it executes.

If you limit the HPWM updates to only when the Duty value changes, it gives a better response.
You can also synchronise the updates with Timer2 to make it "glitch-free".
Sorry, I wasn't clear on my attempt with the HPWM. I set it early in the program to run on its own and would get updated only when needed (when a rider shifts gear). But while using a 55ms timer controlled ADCIN inside a WHILE:WEND, I found it jumping out of the loop way before the timer overflowed because the HPWM triggered it somehow. So I'm sure I didn't account for other hardware commonalities.