Thanks for the reply, Bruce.

Upon further study of the data sheet, I was made aware of the existence of the XINST bit in CONFIG4 bit 6. The datasheet warns about flaky behavior in legacy apps if this bit is set. So I got out the JDM programmer and looked at the configuration bits (using WinPic). This bit was indeed set. Turning off this bit and reprogramming the configuration bits fixed the problem. (re: PIC18F2525/2620/4525/4620 Data Sheet DS39626E pg 314, section 24.2.4).

-Dan