Yep, that did it! The output is now as smooth as the 16F883's output. The 16F883 used the INT_INT interrupt from the start. The GPC_INT requires the particulars you noted, and additionally it interrupts on rising or falling edges (new to me). I had only use INT_INT before prior to this application, but encountered problems when I tried it on a 12F629 switching to the GPC_INT, which I later switched to the 12F683 but kept the GPC_INT in error.

Thanks for the comment!
Dave