Thanks to all that replied.  I wanted to pass along the solution to my
intermittent problem.  I had assumed I had both Vss pins connected to GND.
As I was reworking the proto board to put .1uF bypas caps instead of 1uF, I
noticed the connection to Vss was missing on the one side.  After
resoldering it, the circuit ran fine.

The moral: always check every Vdd and Vss connection on the PIC.  A missing
duplicat connection WILL result in erratic behavior.

Rob