Gremlins...gotta love em'
Well, the other day, it started throwing codes and the SES light came on. I'm pretty sure the codes were bogus and unrelated to the problem (O2 sensor and Evap vent valve/purge solenoid). I erased the codes, and drove another day, with the same problem intermittently occurring. Yesterday, it threw some more codes.
Today, I pulled the ignition switch and tested it. Tested good, so I re-installed it and the problem was gone and so was the SES light.
I must have re-connected something that had developed a poor connection, or my ignition switch is going bad and I just got lucky for now. At least I have a good idea about what it's most likely to be in the event the gremlins should decide to re-appear...
Thanks again for all the input and help.